what is D- from the altenator

I have just brought a 72 type three with some electrical issues. it runs great but keeps draingin batteries, so she alwasy needs to be jump started.

I have the wiring diagram and i am starting from the alternator, it has three wires DF, D+ and D-

I have checked these back to the Voltage regulator DF and D+ are fine, but D- was wired together with DF

Does this sound right?

the engine is out of a beetle and the voltage regulator appears to be for a type three so i dont have anywhere to wire the d- up too

can anyone shed some light on this?