Hi, I have a 2003 Cavalier base model with the only option being satellite radio, but from the factory it had a incandescent third brake light in the back window, well a couple of weeks ago i went to the salvage yard and found a trunk that had the spoiler and LED third brake light on the trunk, so I bought it and installed it on the car and removed the bulbs from the factory third brake light. Well tonight I was out checking my lights to make sure everything was working correctly, I had the hazards on and the ignition on and i looked at the third brake light and it was lit up very dim but only if the hazards were on and the ignition was in the on/run position. If i hit the brakes it lights up normally, I got a voltage checker out and it was getting 0.17 volts at the plug. Is this normal? If it had the LED brake light from the factory did they have a lower rated fuse so it wouldn't do this?

Simple explanation.Your oem brake light assembly had a normal bulb Not a led.The led lights use less voltage and being you swapped over to that you would need to add a resistor inline with the 3rd brake light harness to make it light up as normal.Most auto stores sell these resistors for say 10-12 bucks on average.In short being it uses less amperage is essentially is like it is grounding out or even a bad ground and not permitting it to light up as intended.Any car that did not have led style lighting requires resistors to be installed in order for it to work with your cars electrical as it was designed for the standard bulb voltages not the LESS voltage LED style lighting.This should be correct and if not or left something off may others add or correct me.