Where does the the regulator rectifier mount on my 1976 DT 400.  It has one wire coming out -yellow with a white stripe.   I have heard it called a current limiter for the headlight 

That part is just the voltage regulator - there is a separate rectifier.

If the exploded parts diagram on Partzilla is accurate (not guaranteed!), then it fits somewhere near the horn - look for the corresponding yellow/white wire with a female bullet connector coming out of the main harness.

GR8UNCLEAL Is partially correct as the regulator is separate from the rectifier, the voltage regulator on a 1976 U.S.A model DT400C is mounted on the air box housing 

Thanks for confirming.