Here's my airhorn install:
http://www.cardomain.com/ride/563884/6
Tap the wire going to the horn. It will turn on the relay just fine. Tap the 12v right off the bolt where the battery wire connects to the fuse box. Just use a ring terminal and some high temp / oil resistant insulated wire AND A FUSE HOLDER AS CLOSE AS POSSIBLE. Use that to connect to the relay to turn the power on/off to the air pump.
RJ