depends what year you have 87-91 it shouyld be a 4" newer than that should be a 5x7/6x8 a 6x9 would require a good bit of trimming metal and you will probably run into a depth problem so definately test before you cut
like i said about that, make sure you measure twice and cut once, 6x9 speakers are very deep with big magnets so i would hate to see you cut a big ass hole then the speakers dont fit
Just get better speakers / better placement for your front speakers. there is really no need for rear speakers in a properly set up and tuned system. (unless you are using the bigger speakers for bass, and not running a seperate subwoofer setup).