Q: After I installed a new horn my engine wants to cut out.

I just installed Hella horns on my 2004 Subaru STi. After I installed them and reconnected the battery, I went to start the car. It didn't start up as it normally does. It took longer for it to turn over, and after it did turn over, the rpm's bounced from 2500 down to almost 100. Eventually, it starts to idle fine, but then if a give it a little gas (in neutral) it revs fine, but when the rpm's fall it almost cuts out everytime. It also happened when I took out my old horn, same exact thing. I thought it could be an air intake problem, so I disconnected my MAF, ran the car, reset the computer, and when I took out the OLD horn it was fine. Only with the NEW horns, it did not work. Maybe the alternator is having a hard time compensating for the extra voltage of the new horns? I really don't​ know.

My car has 118000 miles.
My car has a manual transmission.