David said:
I used this to change the speakers in me 2003. Might be the same for
the nearly identical 2004s too.
http://www.subaruparts.com/guides/Forester_2003MY_Speaker_Upgrade_Kit_IxI.pdf
That's quite good but doesn't explain the plastic studs in the doorpanel
very well.
There are plastic barbed studs spaced around the sides and bottom of the
panel that push into mating plastic bushings. To separate them, GENTLY
pry the panel away from the door itself. You may have to try a couple of
different locations to get one that pulls free easily. One you have one
(or maybe two) pulled free just keep going around the door pulling out
GENTLY as you go. It can be done with your hands alone, but there are
upholstery/trim tools that make it easier. Surprisingly, a 3" wide
wooden spatula works well though.
There may also be "buttons", just those same studs with heads on them.
These can be a pain in the ass to get loose. Often a dull butter knife
can be pushed underneath the head and twisted to get it started, then
keep going around the head until the knife blade can fit under the whole
side and (again) GENTLY pull it straight out.
To reinstall, line the studs up (ALL of them) and gently tap the panel
back into place, making sure that you don't jam one out of it's socket.
Going around the door and partially seating all of them is the safest
way. Finish up by thumping them snug with the heel of your hand.
Push in the buttons gently at first but thump them in the same way.
"GENTLY" is the word! Breaking those studs can be expensive, as in
replacing the panel. If you are lucky, you might break one in the middle
of the bottom and get by with putting it back on and it won't rattle or
gap. Sometimes a stud can be fixed with a "glue and screw" repair, but
you will always remember your 'oops' when you see the screw head.