1985 300TD

Driving down the road with heater on - the seatbelt light flashed on and off, at the same time the ACC fan slowed then came back on. Now ACC unit is acting a bit flakey. Fan and vent servos seem to work normally but I'm only getting heat.

Checked fuse box and the fuse for the monovalve, back up lights, horn etc is blown. Had recently put a hitch on car and done a few spotwelds w/o disconnecting battery. Could this have shorted the circuit through back up light ground or similar?

Where to check first? I will look at the ACC board tonight but is there any other fuse that could be blown taking out the circuit?

Thanks for any help-

JB