Any foxbody experts out there? If so, Does '88 mustang t-top cars have side mirrors on side of doors or at the triangle at the pillar post? Want to know because looking for parts on ebay I see '88 t-top cars with mirrors in both locations. Just would like to know which is correct.  

As far as I know, only convertibles and t-tops came with the mirrors on the side of the door, not in the triangular part.  Ive been around fox bodies along time, and unless someone took a door for a hatch or notchback, cut it up and replaced it, you shouldnt have mirrors in the triangle part. Hope this helps you.  Look at the top of the door, if it is just glass there and no metal, thats a door that should have the mirror on the door itself.  The used the same door on convertibles and t-tops too.  

Starting early in the '88 model year, the mirrors on T-roof and convertible doors were switched from the top of the doors to lower on the doors.

So the answer is: both.