1966 Pontiac GTO under dash wiring
 

Trying to decipher what I have. Working on 66 GTO 4 speed console car none A/C rally gauges. The under dash wiring is M&H.. Looking at there website it seems they do not offer a harness for a 4 speed car with gauge package. Closest is automatic w/gauges. Is there one offered for 4 speed console GTO's with gauges? On the left side of the harness there are 3 pairs of wires with 2 wire each and a plug at the end. 2 purple 2 green and 2 orange. I figured the purple are neutral start. Although I don't 66 manual 4 speed had a neutral start. Anyone know what the other 2 pairs are for? Also the diagram refers to a master switch and a master relay.. Where are they located and what do they do? Is that for A/C cars only? I appreciate any input.

The green pair would go to reverse switch . The orange pair would go to the downshift switch if it is an A/T. I belive this is correct and I think you are correct on the master switch and relay.

The purple pair would be the start circuit. On your car there might be a switch on the clutch pedal .

As stated, the purple ones should be the starting circuit. No clutch switch until 69; the factory put a jumper on that plug for manual transmission cars.

Importsmasher

while you've got the car apart it might be a great time to hook-up an underdash kill switch using those purple leads

As mentioned the NS wires would have a jumper installed on MT cars.
Nothing more than a small C-type connector with rubberized insulation on the outward portion.
This would be plugged into the plastic connector to complete the circuit.

Factory Jumper Shown;