Hi! I recently bought a 1984 300D and the reverse lights don't work. There is no power getting to the socket while in reverse. I hope someone can answer some questions for me.

Where is the switch that actuates that circuit?
What is the most common cause of this failure?
Does the engine need to be actually running for the reverse lights to come on? I've been testing with the key in the run position, but not with the engine actually turning.

Any help is greatly appreciated.

Assuming it is an automatic transmission the neutral safety switch (NSS) is on the L side of the transmission where the shift linkage enters the case. The backup light switch/contacts are inside the NSS. The most common causes are the NSS with dirty contacts inside, bad shifter linkage bushings or a misadjusted switch. The switch position is slotted type adjustable.

The lights should work in the run position.

Good luck and welcome! Please complete the thread.

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer

On my '79 300D a new switch fixed my reverse lights and
my inability to start the car with the transmission
in the park position.