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The coupe doesn't have the trunk wire harness issue, that's a sedan problem. Coupe doesn't have lights in the trunk lid like the sedan does.
However it still could have a broken wire due to other reasons. I still suspect you're not actually checking the brake light switch, here's a picture, it's the upper one.
And you still do need to use a digital multimeter to actually test for power, a test light would work for the brake pedal harness too but to test continuity to ground at the taillight you would need a DMM.
The cheapo Cen-tech from Harbor Freight will work find for this kind of stuff, I think it costs less than $10 still. Just set the dial on DC volts 20v range.
Here's a picture, the leads plug into the ports I circles in BLUE, your red lead goes into the top one and that's the lead you test for power with, the black lead you touch to something that's chassis metal/ground. Any unpainted surface will work like a nut or bolt.
For voltage test turn the dial to the GREEN circle, then put the black lead on chassis ground, the red lead tests for power.
To check resistance (ohms) for ground turn the dial to the YELLOW circle, watch the display as you touch the leads together, it will show either 1 or 0 (continuous circuit), put either lead to chassis ground and the other lead onto the pin4 (black wire) of the tail light harness and the meter should show that same 1 or 0 continuous circuit reading.
The coupe doesn't have the trunk wire harness issue, that's a sedan problem. Coupe doesn't have lights in the trunk lid like the sedan does.
However it still could have a broken wire due to other reasons. I still suspect you're not actually checking the brake light switch, here's a picture, it's the upper one.
And you still do need to use a digital multimeter to actually test for power, a test light would work for the brake pedal harness too but to test continuity to ground at the taillight you would need a DMM.
The cheapo Cen-tech from Harbor Freight will work find for this kind of stuff, I think it costs less than $10 still. Just set the dial on DC volts 20v range.
Here's a picture, the leads plug into the ports I circles in BLUE, your red lead goes into the top one and that's the lead you test for power with, the black lead you touch to something that's chassis metal/ground. Any unpainted surface will work like a nut or bolt.
For voltage test turn the dial to the GREEN circle, then put the black lead on chassis ground, the red lead tests for power.
To check resistance (ohms) for ground turn the dial to the YELLOW circle, watch the display as you touch the leads together, it will show either 1 or 0 (continuous circuit), put either lead to chassis ground and the other lead onto the pin4 (black wire) of the tail light harness and the meter should show that same 1 or 0 continuous circuit reading.
my g35 is a coupe model for the record. we are definitely checking the brake switch not the cruise control btw dont worry. the brake lights do turn on so its something before the brake switch. im gonna go see if i can get this dmm today
Ok cool, definitely start with testing the brake switch though, that's the middle point of the circuit so you can determine which direction to keep troubleshooting.
Here's the line direction of power for the circuit it's either going to be
-Bad fuse, no power going to the brake switch
-Good fuse but no power at the brake switch = bad wiring between fuse and switch
-Power going into the switch but not coming out of the other side of the switch = bad switch
-Power leaving the switch but not getting to the brake light = bad wiring between brake switch and brake light
-Power coming into the brake light but the light isn't turning on = bad ground at the brake light.
I'm leaning towards it not being a bad tail light ground since all 3 of the brake lights don't work.
Ok, so the fuse for stop lights inside the cabin fuse box kept blowing every time we stepped on the brake. we disconnected and reconnected the rear "high tail light" harness (the one for the light on the spoiler) and now it magically works. the fuse doesn't blow anymore and the tail lights work. the brake light on the spoiler doesn't work though but I mean I'm still very happy.
Ok, so the fuse for stop lights inside the cabin fuse box kept blowing every time we stepped on the brake. we disconnected and reconnected the rear "high tail light" harness (the one for the light on the spoiler) and now it magically works. the fuse doesn't blow anymore and the tail lights work. the brake light on the spoiler doesn't work though but I mean I'm still very happy.
iirc That rear window light isn't supposed to light and the spoiler light should...but it may be vice versa...actually it probably is the spoiler light that never was hooked up...anyway...
Some folks fool with it to get both working.
The rear window light has a pcb diode that is underrated (1A) and can be replaced with a higher rated diode, but mine stopped working after a bit even then. It is a goofey Chinese design with series diodes and shared resistor pull-ups.
There was a company that made a better designed pcb and occasionally you can find the pcb on ebay. It is just as easy to replace it with an led strip that has separate pullup resistors for each LED and a regulator diode for each section.
The same is true for the spoiler brake light.
After the window brake light died the second time, I ended up unplugging the window brake light and putting in a lightbar inside the spoiler brake light....and sealing it back up with permatex...this was after I tried to fix it by replacing the bad diodes..only to have more fail.
That spoiler light doesn't seem to have any aftermarket support or lights that will fit...do not throw it away!!
You can check the wiring to it, gently pry the back off and then do surgery on it like I did.
If you decide to get both lights working, a little bigger fuse would be ok...
I start the car and everything appears fine. The brake lights light up. Once i press the brake pedal past a certain point (about 1 inch??) 3 lights pop up on my dash: "abs", "slip", and "vdc off". Once these lights show on my dash, the brake lights stop working. When I turn the car off and back on, the lights are gone from the dash but the brake lights still don't work it’s a g35 03 changed bulbs and fuse behind battery is good also changed break light switch still not working I have no idea what it is turn signals work and regular driving lights
Last edited by Tdotg3503; Jul 30, 2025 at 09:51 PM.
