[spoke]

There are 3 spades on the VR, DF, D+, and D- just like the 914. Here's the 914 charging system schematic. The 914 system is similar to the 911 although I believe the 911 alternator has more diodes than the 914 alternator.

Check all 3 voltages at the VR with respect to chassis ground, not the engine case.

D- should be zero volts.
DF will bounce around 5-10V depending on level of charging.
D+ should be equal to the battery voltage.

Measure battery voltage on the POS terminal to chassis ground.

D+ and the battery POS should be equal in voltage. If unequal, then it points to an issue in the alternator.