I wanted to know before I buy it from this guy. There's also a guy with a 1989 Trans Am trying to sell me his, but I know it's not compatible.

Should be the same on all v6 cars from 86-89 .

I believe you won't have the VATS wiring in the 1987 harness. That is one major difference between the 1987 and 1989.

If you need the engine harness then it should work if you need the dash harness I don't remember if 87 had vats or if it is seperate from the dash harness .

Hold on there. Yes, it will work. Yes, the 89 has VATS (if it has the black pellet in the key). No, if you're putting the 87 harness in the 91, it won't matter, unless you have to rewire the starting circuit in C100. The 89 ECM may have a fuel cut-out circuit controlled by the VATS module that the 87 won't, so you're good there.

No, it's not just a direct bolt-in. You'll need to rewire the TPS, IAC, Oil Pressure Sensor/sender, and possibly the injector harness (different connectors). That's if you don't have to redo the dash harness at C207 and the firewall connector C100. I don't have an 89 FSM handy, so I can't be 100% sure as to how different the 89 harness is.