Does fxdb already have a horn relay or do I have to fit one for air horns

no relay in my Fatbob, doubt you have one for the horn in the Streetbob
most air horn kits come with the relay to drive the compressor
I just fitted a Denali SoundBomb to mine and hooked the in-line fuse up to the positive on the starter motor because it was getting a bit crowded in the battery carrier

Strange for Harley, I made bracket for horns with compressor hidden in cowbell & tested using original  plugs for horn & they worked great.

Didn't stuff around fitting the relay as they. work so good even with engine running they are so loud.

Tilted them downwards a bit to try & prevent them filling up when raining.

All up cost was about $30, the horns & compressor were $24 from ebay, a few nylock nuts & a thick piece of aluminium plate.

Wasn't prepared to pay the big dollars that a lot of "Harley" accessory ones cost.

If they corrode or pack up, will just get another set, but the chrome looks ok & I will polish them to preserve finish