just picked up a 91 GT fully modded problem is there is a beeping that sounds like it is coming from behind the hvac controls. the previouos owner said it has been doing it for months on and off. i checked the door buzzer and that was not the problem. any ideas, thanks

 

Its the same problem I have, the air bag system. Does the airbag light flash when you start the car? Count the number of flashes as there are charts for the exact problem. Mine is a bad rear sensor.

 

it flashes 10 times, thanks

 

does it happen when you are backing up?

 

10 flashes indicates a "Firing circuit Disarm Device " according to my Ford Service Manual for 1991 Mustangs. What its saying is you have a short in the deployment circuit and it has a flow chart for trouble shooting the problem. Short version of the flowchart:

Find the short or consult the hotline, DO NOT replace the diagnostic module or other sensors until you find the short. If it were me I would take it to the local Ford dealer asap, you don't want a short to fire or not fire the airbag and the wrong time. But I don't know alot about the airbag circuit to tell you whether it will fire from a short or not. They are however making a big deal about a code 10 for the diagnosis. Yours is the 5th code down on a chart or 1-11 in severity, the furthest down is the least severe so yours is up there a bit lol.

Hope this helps.