89 interior in a 79

It can be done, but of course you will have to work on the wiring and everything for the HVAC system if you want to keep it. A friend put a 92 interior in his 86, using the complete 92 wiring harness, and the only problem he had was in the upper dash mounts. He had a couple of brackets welded to support the top of the dash, but some others have just used long screws to mount it. Most of the 79-93 Mustang interiors are interchangeable, with some mods. But the door handles on the 79 are different. The 87 + rear quarter panel window opening is deeper for the flush windows, so if you want to use them, you would have to either swap out windows or trim the plastic panels. For the tail lights, the actual metal tail light panel is different with different holes cut out for the bulbs. You can make it work, but it would involve cutting the panel up, or replacing it with the newer one. As far as the bumper, sure it will work, but the side trim will not match the trim on the bumper.

I'll be the first one to say don't do it.

Ash