From what I can see on "FO-2. M939/A1 Electrical system wiring diagram (2 of 2)" in TM 9-2320-272-23-5, wire number 57 is listed in two different spots. Both are ground wires, one for the air pressure switch.
My truck had issues with the low air buzzer not turning off even with the guage reading 120 psi. Drove me crazy because it would not happen every time I started it. When it would not shut off, I would crawl under and tap the air pressure switch. Took me a while to figure out which one it was. It would shut off after I hit it. I guess the proper term would be smack the $*%# out of it. I am planning a replacement when the weather warms up. It has not been acting up as of late, maybe a good beating is all it needed.