I recently put the 02' GT seats into my 89' Coupe. Not hard by any means. I used the 02' tracks and they bolted up with no modifications. They sit about an inch higher, but with all the power hooked up, you can adjust it easily.
As far as power goes... The 02' seats use a 4 prong adapter, while in the 89', I only had a two wire lumbar control adapter.
I clipped the 4 wire plug out of the 02' seat, and used the 89' seat plug. This leaves you with two extra wires. Ignore them. You only need to use the thick black wire and thick (?) green wire on your 99+ seats. Hooking it up like this will give you power seats, but no lumbar. You can simply tie in the lumbar controls from the seat into your "new" plug.
Very easy to do. Took me about an hour to get the old seats out, new driver seat wired up correctly, and the new seats installed!
Already had two people comment on them! ( I used the GT seats that have the pony embroidered onto the seat)
~Joey