[tyson42]

The reason for having the relay in there is so the horn button doesn't carry the load of the compressor. Pins 30 and 87 are the switch part of the relay, kind of like a light switch in your house. That switch part of the relay can handle a lot of power, like what your horn compressor needs to run.

Pins 85 and 86 are the coil of the relay. When you connect one to power and the other to ground (doesn't matter which way, just one to + and the other to -), they turn on the switch(30 & 87).

Your horn button needs to either go between the + and a coil pin (85 or 86), or between the other coil pin and -. That circuit can use any 12v+ power, like the one going to your cigarette lighter outlet. And the ground from that circuit can use the same ground as the cigarette lighter outlet.

The compressor circuit goes like this.
from battery + to the fuse.
from the fuse to a switch pin(30 or 87) on the relay.
from the other switch pin on the relay to the + compressor wire (usually red).
from - compressor to battery -.