... my passenger side brake light, recently, has started lighting up all the time, any time I have the lights switched on. I found out from a coworker the other night that apparently it also goes out -- along with the taillight on that side -- when I press the brake pedal. Last I checked, it does come on when I brake and don't have the main lights switched on. So, I must have some kind of short when the lights are switched on. Obviously it's dangerous to have one side of the lights go out whenever I brake, so I want to sort this out ... but I'm not really sure where to start. I didn't seem any obvious problems with the wiring where it attaches to the tail lamp unit. Thoughts?

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1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)

"Clean, Bright and Tight"

Then the usual investigative Drudgery with the DMM.
Worn Insulation allowing Shorts.Etc.,Etc.,Etc.

Pick through the FSM Wiring Diagrams and check Wiring Colors,if anything is not as per FSM investigate

Weird that it's only one side.
The system uses negative grounding.

Anybody done any "restorative" work on that side of the chassis?

Almost sounds like ZOMEZING is amiss inside the Tail light itself.
(Backfeed of Positive current to the B.L. when the headlights are on.)

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'84 300SD sold
124.128

I've seen this type of thing on lots of cars.
any chance someone installed a trailer hitch dohickie?
if not, I'd pull the bulbs on both sides, and clean everything. make sure you have the correct bulb, etc.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Make sure all the earth connections are good at the light. The lights going out when you put your foot on the brake in a sign of a bad earth.

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Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving