I have a question about the wiring for defrost timers in general. from what I've gathered most of them have 4 terminals. 1 for a common feed, one going to the defrost heater, 1 goes either to the compressor or in my case the thermostat first then the compressor and the other looks like it might monitor the compressor or maybe it is a ground.

I want to be sure about that last wire and whether or not the unit would require a ground to operate. The wires I have available are a constant live feed, a feed to the defrost heating element, a feed to the thermostat which should only be drawing power when the fridge is running but is always powered up except when in defrost mode and I also have a wire that is tapped off of the compressor which is only live when the thermostat is on but the power doesn't pass through this wire it is merely piggybacked off of the thermostat.

that 4th wire is either to be monitoring the compressor which doesn't seem to make sense because power isn't passing through the timer to this wire but merely monitoring it. being I don't see a ground wire anywhere I would imagine the timer would need to monitor the power that has to pass through the timer which is the wire going to the thermostat. if someone could confirm or correct my understanding I would greatly appreciate it. thanks.