Welcome to the forum, Hans.
Since this is an '87 seat speakers aren't an issue unless at some point you swapped in '84-'85 seats (or recovered and modded your stock seats) and wired in their built-in speakers.
Do you have the performance sound option? A quick look at the dome light will confirm, if you have a switch labeled "Performance Sound" with sliding bass control then the answer is"yes" and at this cars age its almost a certainty that the subwoofer speaker will also need replacement, there are viable replacement options available.
The stock front enclosures are quite deep so just about any 4x10" speakers will fit without problem, or if you are willing to consider not going with just a plug-and-play solution you could look into using a component setup and fabricating your own mount. Pioneer and Kenwood among others still make drop-in replacement speakers that will fit.
The bigger issue is that the stock location for the rear speakers is pretty shallow so your options are going to be limited. I used Pioneer TS-P461 4x6 plate speakers many years ago which were thinner than most, but which also barely fit. From memory the absolute maximum thickness you can get away with is either 1-3/4" or 1-7/8". Nowadays you might need to come up with a 4x6" adapter and use sufficiently thin 4"speakers.
Also if you haven't figured it out yet you access the rear speakers by loosening and pulling outward the section of trim. If you try to just pop off the rear speaker covers you will break them.
You can also use the search function and find out a lot more on this topic here.
[This message has been edited by LarryB (edited 05-18-2008).]
