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[yarisugi]

Compressor has two wires - one ground, one 12v. The 12v wire connects to a relay.
The relay has 4 wires - one ground, one that connects to the compressor, one 12v to battery and the last wire connects to the pressure switch on the tank.

When the tank drops to a certain psi, the pressure switch sends a signal to the relay which triggers the compressor to turn on.

You probably don't need a relay, you can hook up a manual switch and turn it on whenever your tank runs low. However, you'll need a gauge to monitor how much psi is in your tank so you know when to manually turn the compressor on and, most importantly, know when to turn it OFF so you don't overfill the tank beyond its holding limit. Don't forget to add a fuse in between the battery and the manual switch just in case.