1999 Oldsmobile Cutlass My driver side brake light doesnt w

Please help me understand the problem.
You replaced the bulbs and checked the fuses. I have color-coded your tail lights. Driver side you have red and purple, passenger side blue and orange, and the high mount stop lights green rectangle.

With the engine on, turn on your headlight.

1. Without depressing your brake, are the tail lights working? All red, purple, blue and orange. If no, which one working? If yes, go to 2.
2. Step on the brake. Observe stop lights. Using color codes, describe the ones that are working.

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Thank you.

Seeing that there is a cross between exterior lights, turn signal and back up light circuits (engine on, brake and reverse on; driver brake light off, driver backup light off, driver turn on), I can tell you that your junction block is short-circuiting. You may have to replace the entire unit.

It is the fuse box on the passenger side. I believe the box itself is short-circuiting. If you look at the wiring diagram, the wires have different numbers on them, but close to one another. You may want to remove the fuse box from its bracket and try to see the back side of it for possible short-circuit. I don't believe that you can remove the harnesses. It probably just need some cleaning with some electronic parts cleaner such as CRC QD

If that does not work, you may have to search some wrecking yard. I tried to search eBay, they only have a listing for under hood fuse box.

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1. It is possible to check continuity. Use resistance function. Pull the park lamp relay (under hood junction box, left side of engine). Check continuity between 85 and 86. And check for no continuity between 30 and 87. Turn the key on. Check for continuity between 30 and 87.

2. Your relay is under the hood, not at the passenger side junction block.

3. I don't think it is relay this time, as the passenger side lamps are working properly.
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Since the passenger side fuse box is good, the problem must lie towards the back side of the car. I spent quite some times reading through the wiring diagram to see correlation between you tail lights, stop lights, turn signal lights, and back-up lights.

Reason:
1. When you step on the brake, left tail lights dim out, left stop light don't work, left turn signal turns on, both back up lights on, left 3rd brake light not on, and left side side marker not on.

2. When you put on reverse, left tail lights not on, left brake lights not on, left turn signal on, left back up light not on, left 3rd brake light not on, and left side marker not on.

Your back up lights get power from back up lamp fuse 10A, goes through the neutral safety switch on top of the transmission, to B11 and C2 in the passenger fuse box. B11 switches to B2 to the left back up lamp in the tail assembly, to G314 ground on left side of rear package shelf. C2 switches to C3 to the right back up light, to G313 ground on right side of rear package shelf.

Your tail lights get power from park lamp fuse 15A to the multi function switch that negatively triggers the park lamp relay through the body control module. The relay splits power to 5 different wires:
1. Right front park/turn
2. Right front side marker
3. Left front park/turn
4. Left front side market
5. Right passenger side junction block. This junction block then splits into 3 wires.
1. License plate lamp and grounds to G314
2. Right rear tail/stop lamps and side marker. All this grounds to G313.
3. Left rear tail/stop lamps and side marker. All this grounds to G314.

Now, as all the lamps are negatively triggered, ALL bulbs have to be working. If even one bulb is not working, all systems will be affected, such as yours. As the license plate was blocked off, I could not see if your license plate light was on, and that your left 3rd brake light was out. I would concentrate on those two bulbs, all left rear bulbs and also the ground coming out of the left tail lamp assembly.

Cool.