An easy solution is to take and re-wire the compressor and relief to a SPDT switch with center off.
One position would activate the compressor.
Center would be off
Second position would activate the relief solenoid.
View attachment 95017
Disconnect both connectors shown
Connect solenoid ground wire.
------------ Compressor
+12v ----FUSE ----- SWITCH
------------ Solenoid
I'd have to dig a little deeper to tell you what A for the fuse but I'd likely start with a 5A unless someone knows more specifically.
An as mentioned, do contact Axel as I think this was something added later on to his modules.