Turning key to "run" causes all power loss.

How much current, if any, went through wires for the volt drop test from the battery to alternator?

If your car has 8 gage wire, then it has the 'fleet' or the optional heavy duty wiring that came with rear defrost grid cars. The charge path is a bit different for each of the three possibilities. Lets try to establish which of these the car has before trying to pinpoint the problem location(s).

Posting photos will make it easier for us to help.