Welcome to JustAnswer.com! My name is DriveFast71(Ron) and I will do my best to answer all of your questions completely and accurately.
The engine controller is located in the engine compartment. It is mounted next to the air cleaner housing, next to the inner fender.
Unfortunately, due to the age, i'm not seeing a "testing" procedure. Here's what Jeep says:
No specific procedures are provided to diagnose the ECU by itself. ECU failure is determined as a result of a related system failure.
Like I say- inspect that wire harness completely. Most commonly, there will be a loose or broken wire at the harness as it plugs into the ECU. If not, then somewhere in the harness there is a "rubbed through" wire, or a wire touching metal, or two wires touching or even a faulty ECU itself. From here on out, unfortunately, it's just a lot of digging and visual checks.
Oh boy... good question... i can *TRY* to see what I got... but no promises. Can I get back to you in the morning, as I was actually just leaving for the day...