1999 a4 horn keeps going off when i turn wheel mainly.

There appears to be a short internally in your steering wheel.

Possibly an errant theft alarm??

If you stop the horn then your car is no longer road worthy. You have a short circuit, most probably by the steering wheel itself where the wire to the horn connects the horn button. Remove steering wheel and inspect contact strip first.

Are you saying the horn blows when you turn the steering wheel ?
Could be the horn pad on the steering wheel, or a wiring problem in the column.

Common 1999 911 problem Stephanie that needs to be repaired by a qualified Porsche mech. He will need to remove face of your steering wheel and replace horn switch that has worn out and is loose inside steering wheel. Will only get worse and could cause you issues when you hit your brakes behind car(s) stopped for red light. Trust me - I almost got in a couple of fights as they thought I was being a wise guy & "playing Porsche games".
Lew Andersen

the reason it is doing that is either the pad on the steering wheel is shorting as if you are pressing on it, or the clockspring under the steering wheel is shorting to ground. there is a horn relay in your fusebox inside the cabin; you could disable it there by pulling out the relay. don't pull the fuse because it carries other circuits as well. I couldn't find the exact relay position, but just feel them with your finger while someone honks the horn; the one that's clicking with the horn activation is the one to pull out. I can get you the layout of the fusebox but you really don't need it for that.

The ground to the horns is the metel bracket on the horn itself that bolt to the frame of the car , also check the contacts for rust or corrosion , also worse case it might be the clock sping behind the steering wheel try pressing the horn while turning the wheel hope this helps good luck

TO REMOVE CRUISE CONTROL SWITCH. FIRST TURN OFF RADIO TO KEEP FROM LOSING CODE. DISARM AIRBAG IN THE STEERING WHEEL BY REMOVING NEGATIVE BATTERY CABLE WAIT 20 MINUTES FOR AIRBAG TO DISCHARGE BEFORE REMOVING AIRBAG AND HORN PAD IN STEERING WHEEL.ONCE AIRBAG IN STEERING WHEEL OUT.YOU CAN REMOVE THE CRUISE CONTROL SWITCH.BECAREFUL REMOVING AIRBAG IN STEERING WHEEL CARRY IT AWAY FROM BODY WITH AIRBAG HORN PAD POINTING UP.IF YOU DROP IT.DONT PUT IN BACK IN STEERING COULD DEPLOY ON INSTALLATION.

Simply have it replaced, horns on domestics usally stop working as mine did on my 93 lumina, it stopped working and I had it replace and it worked fine after that.

problem is under the steering wheel,if vehicle has air bag its best entrusted to a garage,if it hasnt then just pull steering wheel off with a special puller and check the carbon brush and the wires and contacts in the steering wheel itself.This is a simple job but without the right tools ???