Im trying to install an ampilfier and i cant find the firewall on my 1985 cutlass supreme.

The firewall is what the brake booster and a whole lot of other items are mounted on in the engine compartment. It is the portion just under the windshield. If you are trying to get to it from inside the best place to get to it is where teh wiring harness comes through on the left side near the steering wheel.