More often than not, the issue is poor grounds in the system. (Although that may not be your problem, as you've already tested, and failed, for +12V at the brake lamp switch.)
Sounds like you may also already have the FSM; but if not, you can download it for free for all years at: https://www.xenonzcar.com/s30/fsm.html. (For your ease of reference, the stop lamp circuit is attached as a one-page pdf.) Hopefully I'm not giving you info you already have. π
Stop lamp circuit.pdf