I'm trying to set up another horn in my car, and I have a few questions. Firstly do I connect the power supply of the relay (terminal 30) to my cars fusebox or the batterys postive terminal? Also how would I set it up so that I have it connected to a switch that selects between my old horn and new horn. Pic related
Terminal 30 should go where ever it's easiest to get voltage. But, I would find a switched fuse block not being used. Either you have a second switch or have a 3 way switch so you can use your steering wheel's default horn location.
You'll need a switch between either ground or 12v that chooses either old horn or new horn. Or both. Or none. You'll figure it out.
Are those the two horns you want to select in your pic? I would suggest using a seperate relay for each one. Connect both terminal 30s to the fuse. Connect each horn as shown in your pic. But install a SPDT toggle switch between your horn switch and each relay's terminal 86. That way, the selector switch will only have to carry the low relay coil current.