Alarm system tampered with by previous owner

[Dutchvelvet]

Excuse this if it is a repeat. I saw a lot of alarm posts but couldn't really see a full blown wiring diagram for my car. After buying this 92 Supra, I discovered afterwards the dome lights and radio did not work. Prying into the under steering will enclosure, I discovered a computer that was disconnected entirely from the 12 pin? harness and a several wires bunched together with a wire nut. (can give all the wires color codes when I get to it) When I attempted to plug the computer back in, of course the alarm went off. The car runs and starts perfectly so I don't want to try to get the radio working only to disable the starter or short something out. My Haynes manual has not come in the mail.
Today we are starting a work day on this project car with lower and upper control bushings going in so we will be working on it already and thought we could fix the radio and dome lights if someone had the wiring diagram and any hints as to potential problems that could occur if we (my son and I) pry into this thing. Let me know if anyone can help