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Hi all - We recently discovered on our 79 that the lower rear tail lights dont come on anytime and the side markers dont either. The uppers work and the turn signals and brake lights all work fine. Are these other lights on a different ground or circuit altogether? Any thoughts? Th wiring is untouched original.
 

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I didn't think the stop/tail/turns were multi bulb setups. From what I recall all that's there is one S/T/T and Back-up lamp on each side. Are you looking at the reflector and thinking its supposed to light?

+1 for checking the side marker lamps.
 

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There are 2 bulbs in the tail lamps. Frankly I assumed the rear side markers lit up since the fronts do. Anyone know if the upper and lower rear light bulbs are on a different circuit?
 

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Upper lamps are single bulb dual filament, working as turn/tail/stop. Lower bulb is single filament for reverse lights.

Side markers have lamps that are wired in on a circuit spliced to the tail in the rear and the front markers on the front, and operate when the headlights are turned on, but do not blink with turn signals.

Normally the lights are completely off even with the engine running. (No DRL on these old beasts)

Pressing the brakes, using a signal, or turning on the hazards will make the upper tail turn on.

Turning the headlamps fully on, or to parking lamp on, will make the upper rear turn on dimly, and will turn the sidemarkers on. Pressing the brakes or toggling the signals will make the upper tail lights blink brighter.

Shifting into reverse with the key in the on position will light up the lower lamp, regardless if the engine is running.


When I first got mine the side markers weren't working. Diagnosed as three problems over time. First the ground side was bad so I had to re-secure the ground bolt. It's behind the tail light housing on the body. Then the sockets themselves were too corroded. The contacts were bad. I replaced them with new sockets (and added LED lamps instead of factory bulbs - same socket, just a different plug-in lamp). Then I overhauled the entire wiring system and found that the original wires were chewed up pretty bad in the rear altogether, so they didn't have much life in them.

The splices are all in the rear, accessible from under the rear bumper or if you are lucky through the tail light housing holes. They are wrapped up really well in oem cloth tape so are a little difficult to spot at first.

If the reverse lamps don't light up and the bulbs/sockets are good, it could be the neutral safety switch on the side of the transmission which needs replacing.
 

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There are 2 bulbs in the tail lamps. Frankly I assumed the rear side markers lit up since the fronts do. Anyone know if the upper and lower rear light bulbs are on a different circuit?
One of those bulbs is NOT a tail light. Its a reverse light. So to answer your question, yes. The upper and lower bulbs are on different circuits because they perform completely different functions.

The side markers should light up as well.

MDG explained it perfectly.
 
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