I've got a '90 wagon and I replaced those a couple years back. Hmmm, if I remember correctly I took the following steps to get to them:
1. Remove trim at rear door (top and bottom)
2. Remove nearby hooks (at sides of door opening)
3. Remove back seat side padding (both sides, basically to expose part of wheel well trim)
4. Remove side window trim panels (be careful here, I broke some tabs and I've regretted it since)
5. Finally remove wheel well trim panels (you'll have to raise the spare wheel cover) and voila! the rear speakers are exposed.
Now I know I've not been as detailed as I should have, but that's about the gist of it (I hope I haven't forgotten anything). I should have some diagrams in a box somewhere. I can't promise anything, but I have a look on Sunday.
The tweeters on the ceiling are easier, just slide a flat tool (panel remover or putty knife) under them and carefully pry them up...I can't remember which side the tabs are on. I can check later.
I think the stock head unit powered 4 watt speakers up front, and 10-12 watt in the back...but I'm not sure

, my memory isn't that good beyond a few months

They were all 4ohm though, that I do remember.
Hope I've help out a little, I'll try to get more info for you. I had fun replacing my entire audio setup, so should you.
Cheers