More specifically you want the TEWD (Toyota Electrical & Wiring Diagrams):
http://www.cygnusx1.net/Supra/Library/TEWD/MK3/
The Toyota Service and Repair Manual (aka TSRM) is here:
http://www.cygnusx1.net/Supra/Library/TSRM/MK3/
They're both for '89 and newer so if your car is an '88 or older there will be some diferences.