First:  This is a pretty difficult job, if you have no talent for this (nuts, bolts, soldering and stuff) don't do it.  You can damage your stereo and car electronics or scratch or crack the plastic frame around the door.  I am not responsible for your actions or inability.  This job should take an hour or so.  You'll need special tools, a T-wrench (not sure what size but it's small and used for removing the metal trim around headlights), a small stub phillips screwdriver (that's the +  or cross type one the call in the U.S.),  and a regular flat screwdriver and tiny flat screwdriver, needle nose pliers, blanket (for when the screws drop and comfort), jig saw to cut the backs, jig saw, drill and a drill bit to enlarge the holes, speakers (I got 4 Polk speakers (6 1/2"}on sale from $97 to $69 and a second set half price.)    They can't handle the bass I want but the stereo has outputs to a sub woofer.  Depth of speaker.  I needed to cut mine but if you get some real deep speakers.  Be careful as your window needs to go down in the door and might hit it.

I only had the key off when I did this.  Others say to disconnect the battery.  That resets EVERYTHING, car computer, alarm (flashing parking lights) would need to be reset (turn ignition on/off 5 times) and remotes programmed again (maybe).  I just keep the key out of the ignition.

Do this in the shade as looking up to the bright sky will kill your views behind the dark trim.  This is a shot of the speakers.  Image was trashed but I salvaged most of it.  This shows the original BACK speaker and the new two-way speaker.  The original speakers aren't that bad, foam edges but a cardboard "wizzer" cone.  The speakers are mounted on  plastic mounts.  The mounts come off first (three large phillips screws) then use the T-wrench (in picture, see the small tip) to remove the speakers from the mounts.  

The holes do match if you move the speaker around the holes.  They still aren't perfect but you may want to drill out the correct holes by pushing in towards the center of the speakers to make the holes shorter from one hole to another.      BE CAREFUL BECAUSE IF THE DRILL BIT SLIPS..IT WILL FIND SOMETHING TO DIG INTO, MAINLY BODY PARTS.  And use a bit about the same size as the hole.  Pressing on the side of a drill can break it.  Only need a tiny bit of extra metal taken off towards the middle of the speaker.

I was able to get the rear speaker screws in by putting in the T-screws at an angle until they caught and straightened out.  You could drill different holes, it's only a plastic mount.

Subaru 2001 Stereo Pin printout.

First you lay down the blanket, SAFETY GLASSES on!  Should something drop into your eyes as you are looking up, pow!.  [They only cost a buck or two and the emergency room deductible is $50. Think about that!  You need your eyes to drive and see the stereo buttons.]  From the bottom center of the inside door use your flat screwdriver and carefully pull the plastic trim away from the door.  There are white plastic "rivets" that hold the door on.  If you loose or break them, see your local dealer or auto store for replacements. The rubber seal is also held on by the edge of the plastic trim.  It may fall off.  Beware of  the black sticky glue holding the clear plastic to the door metal.

This photo shows the door handle screw.  From the bottom of the plastic door tab,  pry it up carefully.  The square plastic rivet will pop out.  A phillips screw is in there holding that section of the door tight against the frame.  Save both and note the "TOP" side on the square rivet.


 

I used the box the speaker came in to hold the bottom opened once the center door rivet is out.


 

Speaker coming out
Use your snub (short) phillips screwdriver to get out the three screws holding the plastic frame and drop the speaker out.   You don't have to remove that green connector.  The contacts are on the back.


 
 

Find the speaker wire connectors.  Wiggle them off, don't break the white plastic mount. If need be, use a flat screwdriver to pry them apart carefully.  They have foam tape around them.  Save the foam by slowly pulling it off.    In the door again,  you need to take out one of the connectors with a tiny flat screwdriver as most replacement speakers do not have these matching connectors.  See the blue dot.  That's the + on the speaker.  On the speaker connector you can take it apart and pry out the (negative) connector.  It's a hassle but the connector does fold back then apart.  I kept the + in the original holder and put the small speaker contact on the edge of the large negative connector.  If you put the large factory connector in the middle of the smaller speaker negative terminal, it will fall off from vibrations.  Have this connector go on the edge.  You will need the needle nose pliers to push on the connectors.  The flat screwdriver is used under the connector so you don't press down too hard and crack the holder.  The T-screws will fit on the back speakers.  Maybe you just want to drill a new or new holes.  I put in mine at an angle until they all fit, then screwed them in straight.  You could drill out the speaker holes to be oblong giving you that extra space.

Second job is to cut the plastic Splash Guard so the bigger magnet will fit in the holder.  I used a jig saw and still have 10 fingers attached to my hand.  As you can see from my web page I have added additions and a 16'X34' foot deck to my home so I am careful and skillful.  You could use a just a hack saw blade to make the cuts.  This allows the speaker to fit but still protects the speaker from water going down the window.  Seal the cuts with silicone glue or the like. 
(This shot shows you the large (+) contact and smaller (-) contact.) on the new speaker.


 
 

Replace the speakers after you put back the contacts and use the foam covering.  Screw them on the door.  The BACK speakers are pretty easy.
The plastic trim is whacked back with your hand making sure the rubber trim is attached to the plastic frame and the white pins are in the holes.  The square plug goes back after replacing the screw, noting the TOP on the plug.
 
 

FRONT SPEAKERS:   You do the same thing to get the bottom off and handle screw out.  There is a plastic plug by the door hinge.  It will pry out, with two flat items, one on each side.  But something doesn't let it go back tight.  Looking into it.  Maybe something fell out.  These plugs are usually quite simple.  Try not taking those out but not much room is available and you don't want to create a crease in the door cover.  After removing the foam cover, this photo shows the plug that must come out by putting in the screwdriver in that notch to release the lock on the plug.   You must remove the connector to get the speaker out as the connection wires are short and little space is available with the front doors.  The new speaker doesn't have the same clip that holds the wires to the speaker frame.  My front speakers don't rattle without that clip.  This mount (in the background) points the speaker up a bit. The wire goes between the speaker and plastic mount.  There is a notch for this.  Make sure it goes in correctly.  This one needed the back cut and the speaker holes moved inwards via drill bit.  As you will see on the speaker connector in the door these speakers must be wired to the tweeters if installed or not.  This is a 4 ohm speaker so the tweeter could be adding another 4 for an 8 ohm system.  The back speakers are 8 ohm.  Try to find 8 ohm replacement speakers, very hard.  I had my 4 ohm speakers in for 2 days until I installed the new stereo.  Unless you really crank it for a long time, you shouldn't have a problem.  Plus you won't crank it as 10 watts doesn't sound good at loud volumes.  But, I don't think factory stereo have all the safety features (short protection, low ohm protection, overheating/overdriving protection) as replacement stereo.  Just a warning.  Don't blame me for a fried amp.

With the speakers back in, give it a test.  The grills are part of the plastic trim so you shouldn't get any rattles.

Tweeters. A few months later I purchased factory matching tweeters for $75.  Fit perfectly.  Pry off the door plastic spacer, put in the tweeter with the factory plug.  In back business in 15 min.  They make an improvement.  https://subaruparts.com.  (in 2021 - Amazon, Ebay and others)  You might be able to get standard tweeters for a lot less.  You pay for "factory fit"

 I was able to get a rear hatch handle (gotta have - check hole distance twice !) and factory painted wing (holes did not match up but fixed it) that match perfectly from Subaru-parts.  I think one was a few bucks cheaper with less shipping costs.  They are still $$$ for fiberglass wing but the paint match is perfect.