Thermostat Wiring Help Needed, PLEASE

[211Homestead]

Howdy All.
I am in need of basic electrical help.
I have reached the stage in my Class C rebuild that I am installing a new AC unit and Control Box (Coleman Mach 15) and Thermostat. The AC unit will not turn on WITH ONE EXCEPTION. It runs perfectly when I touch a 12v Circuit Tester probe to one of the thermostat wires where it is soldered to the Thermostat board. This way I can get both fan speeds. Not sure if I can get the compressor to run. The other end of the Circuit Tester is clamped to a negative terminal on the house batteries.

I'm just smart enough to know that I'm completing a circuit by touching the grounded probe to the Thermostat's soldered wires. Thinking I'm missing a ground-wire somewhere, but all six Thermostat wires have a home, with RED being the 12v load.

NOTE: The Onan 4000 generator is running when I do all this. There should be Amps to spare.

Thanks in advance for your help.

[Old-Biscuit]

Here is a service manual that might help..........
https://www.rvupgradestore.com/v/vsp...e%20manual.pdf