Status report - still no joy . . .

Here's where I'm at:
Alternaotr - tested and good - also shows 13.8 when engine running.
Fuse link wires - tested and good
Batt idiot light works off alternator.
Still get 12.5 when engine running.

I didn't note this before, but I have a kill switch on it (rally car).

So here's how my setup is:

Battery in trunk grounded to body and pos line goes into kill switch. Other side of switch is two lines - one to starter and one to fuses on engine bay. Then pretty much standard from there. I get the 12.5 volt reding at the fuse box in the engine bay and cabin.

When engines off - I get the battery reading it's 12 volts and same at fuse box and alternator.

Somewhere I have a bad cable- which one????? Car worked fine for two years otherwise - now just won't get the voltage right. . . .