Hey mate,
I just finishing installing a new head unit for my 92 laser and am putting rear speakers in. Head unit was no probs wiring diagram can be found here:
http://www.installdr.com/
the speakers in the door are 6.5 inch I think, but just use the cut out found on the back of most boxes the speaker comes in when you buy it.
As for the rear speakers, well I've been having troubles there. The little plastic covers on the back come off but dont' provide very good housing for the speakers. I am thinking of buyin a divider from a wrecker or something. Or possibly cutting a whole around the plastic on the flat surface as there is already a roughly 6 inch circle there. (Hope you know what I mean lol)
Dunno if this helps, if you find a better solution for the rear post and let us know