I don't have any power at the compressor or low pressure switch. If I jump 12 volts into the compressor everything seems to work. Where should I start to look?
Start at the compressor and work your way back until you find the beginning of the power loss ... by running 12V straight into the compressor, you know the ground is good.
ie ... if you turn it on and power goes into a relay, but does not exit, there in lies the culprit