I know in the car I got a harness from years ago it was under the dash pass side. But be thankfull it doesn't go off. It's really annoying. If you hear a rattle comming from you motor , get out of the throttle quick , it's leaning out. You should be alright at that boost level though. If you want to add more boost you should get an adjustable fuel pressure regulator to increase fuel pressure. Are you familiar with www.turboford.org ? That's a good place to talk to the best experts around.
On my older 85' the plug for it is under the hood right there by the EGR switch on the passeger fender pretty close to the fire wall. (Its kinda brown and tucked way down under my EGR switch and not to be confuse it with the EGR vavle on the TB on the car) My plug is a one wire and some one cut it off before I bought the car. If you really want to hear the buzzer, find the vaccum line from the gauage and disconnect it from under the hood were it plugs in to that vaccum rail. Cut the ignition on but don't start the car, then plug a air hose up to the vaccum tube going to the gauge with a air gauge and messure out 18-20psi going to it and you should hear is buzz. If it was me and all you are going to run is 18psi I would make sure the buzzer works, it could save you some money later on down the road. The buzzer isn't to loud, but it could get old if you hit it enough times