okay, well I just recently swapped on 89 mirrors on my girl's 86.5, and didn't have to change the triangle mounts..
however the plug was different, it went in, but there is nothing to hold it together (clip's don't match up), the mirror switch moves the mirror, but moving the switch up, moves the mirror right, and moving the switch right, moves the mirror down, and so on...
the screws were a little too long, so I cut about a 1/4 inch off with a hack-saw for the passenger mirror (no idea why it wouldn't fit) even if I used the screws that came with her original mirror...
driver's side was fine, but the passenger mirror was different, just a heads up, might be different for 87/88 Supras..