Replace/fix car horn? 96 Corolla

I need to sell our 96 Corolla that has basically been
sitting in our driveway wth very little use for a year
and a half. Problem is right before we stopped
driving it the horn started randomly going off when
turning, and at one point it went off while I was in a
parking garage and wouldn't stop. A nice older man
who had a tool kit in his truck helped me
disconnect the horn. We've never gotten it fixed. I
could just go get a new horn and plug it in, but I'm
worried the same problem will crop up. Anyone
know if this is an easy DIY or not? How much can I
expect to pay to have it fixed at a shop, assuming
there's weird wiring going on?