Still trying to rewire my car with a "universal" replacemnent harness. I have run into something I cannot understand about the igniton switch wiring.

My 64 is an original SS guage car, the terminal marked GRD on the back of the ignition switch had two green wires attached to it originally. One went to the temp sending unit in the intake manifold and the other went to the temp guage in the dash. Anyone know why GM would not just run a wire directly from the sending unit to the dash? Is the ignition switch being grounded through the sending unit? Do I need to rewire this same way or can I run a wire directly from sending unit to temp guage. If I bypass the ignition switch with this wire do I need to run a ground wire from the GRD terminal on ignition switch to the body?

This green wire to the switch is for lamp check on an dash light car. Not sure if it's required for a gauge car (not "really" required on a light car either), but it may do something like zero the gauge before the engine starts. The switch connects this terminal to ground when the key is in the "start" position; so that running a jumper straight to ground doesn't accomplish anything.

Anyone else? Don't want to proceed without understanding why GM chose to route the temp sending unit wire through the GRD terminal on the ignition switch.