1969 Buick Skylark Electrical Voltage Question. Stumped

1969 Buick Skylark V8 47000 miles

Recently picked up a fully restored 1969 Buick Skylark GS400. According to previous owner all new wiring harnesses, everything replaced or rebuilt. New Voltage regulator, remanufactured alternator from Year One and new starter. Car has aftermarket electronic conversion in distributor.

Problem:
Put a new battery in the car and approx. 1 week later battery indicated low voltage (around 3v). Almost fully discharged. Got a new battery as it was under warranty. Nothing obvious in terms of loose connections at any major components. Nothing that I could see was left on.

Here is what I have found so far:
When I put the new battery in I disconnected the negative terminal and left the positive terminal connected. I then bridged between the negative terminal and the negative battery cable and got a read of 12.6v on my volt meter. This is with the car off, key out, nothing on. Should I be getting any voltage when I do this bridge?

Second thing I did was check for amperage draw. I reconnected the negative battery cable to the car and put a clamp amperage gauge on and got a 0.0 read. So no obvious draw. I then opened the door of the car and it did show amp draw of.95 as the inside lights went on. So its clear that I am not getting any amperage draw with the car off.

So I am not understanding what is going on and what my next steps should be. Should I be seeing any voltage if the car is off and I put my volt meter across the negative battery terminal the battery cable? I was going to begin pulling fuses one by one to see if I can isolate any change in the voltage. I was also going to begin pulling off connectors to the voltage regulator and the alternator to see if this is causing anything.

Any input, suggestions, ideas? Thanks in advance
Rich