I have a 1995 Toyota Corolla. Going by what this thread says:
http://www.toyotanation.com/forum/t64414.html
I should be able to put 6.5" speakers in the 4" stock hole. I went out and bought a pair of Polk DB650 speakers today but when I went to put them in, I couldn't.
Depth wise, the speaker fits, but there is a plastic piece that the speaker screws onto that will only fit a 4" speaker (the screw holes are too far in).
If anyone knows of a new plastic piece for 6.5" speakers I can buy, or something I can do to make it fit, I would be very thankful.