2002 Oldsmobile Alero horn and turn signal problems

We had a transmission put in this car by a mechanic and now when you press the horn it doesn’t work but instead the signal lights come on, the wires appear to be hooked up right, no crossed wires, any clue?

RE: If this started after the trans replaced, then it sounds like there is a wiring harness smashed and wires are shorting together. That is why operating one component makes a different component function.