Did the 5 pin relay come with the kit?
Normally in a 5 pin relay, its set up as follows:
85 - ground
86 - + from switch
30 - to whatever it is thats being turned on (in your case the tank/compressor)
87a - source a, 12v in
87 - source b, 12v in
The idea is that when the switch is in the off position, the circuit draws power off source a, but when the switch is flipped on, it energizes the relay, and allows the power to come from source b instead of a. For example, i have a flasher hooked up to my mirror lights, so they strobe. When i have the flasher unit off, they act as normal running lights. When i flip the flasher on, the relay energizes and switches the input source to the bulb from the normal running light power a over to the input from the flasher b.
That being said, I believe if you leave the 87a pin empty, in the off position, it wont recieve any power.
Ground to 85, 12v from switch to 86, 30 to compressor, and 87 from battery.
Also, on your switch, by black, do you mean ground? Unless the switch has a light in it (which would mean it has 3 pins), it doesnt need ground. Red power goes in one pin, and out the other.
I have never wired a horn set up tho, so i dont know if yours has directions or not. That above is just what i know of general eletrical stuff.
If im wrong, someone say so and ill delete this lol