‘66 Cuda Won’t Turn Over - Only Got 6V at Switch

There's a couple things you can do. This is surely a short "but where?"

Things to think about..........

Harness wiring often gets welded together when heated due to a short, causing crossover connections in the harness. This can do more bad things and really throw you off the track

One thing you can do to protect yourself is to put a series limiter in the battery ground lead. This can be a small auto -reset breaker maybe 10-15A or a big lamp, like an old headlight, or a stop/ turn bulb with the filaments wired in paralled. With such a thing in the ground lead, you can dead short the battery side to ground, and all it will do is trip the breaker or light up the light.

Check the yellow start wire for shorts to ground. Pull the connector off the ignition switch, disconnect the battery, and disconnect the yellow start wire from the relay. Check that wire to ground. Should be open.

Now check out the brown / bypass wire same way. Disconnect loads on the ignition buss. Pull the alternator fields off, pull the connector off the VR, disconnect your ignition system from the feed. Try to identify anything else fed by the brown bypass (ign2) when in start. Check that to ground with a meter

With your safety light in place in the ground lead, and the key held to start to "activate" the symptoms...........might need two people........try a wiggle test of various places in the harness. Under dash, under hood, etc, look for dimming of the bulb which indicates "you are close." Make these tests with the yellow start wire unhooked from the relay, and as much load as you can removed from the ignition buss.

Look the harness over for "melt damage" on the covering.