I had the exact same problem 5 years ago. Took it to a shop and they said it wasn't the bulbs but the device that the bulbs plug into that was bad. They claimed that GMC used "cheap parts". They fixed it (it wasn't cheap) and not even a month later, the other side went out. They fixed that as well. Now, 5 years later, I'm having the same problem again. Been searching this site for answers. Was going to start by trying to replace the bulbs myself, but I can't even seem to get the old bulbs out of their sockets. Is there a special tool you are supposed to use?
I had the same problem with my 1987 Trans Am GTA. The turn signal indicator has gone bad and needs to be replaced. That will get the brake lights working again.
Turn left indicator signal flashes rapidly when I use indicator, bulbs ok, no lights working
If you are getting an indicator of a burned out light, but don't see a burned out light, it must be a turn signal light. On a Mercedes, you'll actually get a "burned out light" indicator if the turn signal is not working.
Did you happen to check the turn signal flasher before you installed the switch?
When the signal lights are not working on a car, there are a couple of things that could be going on. One could be that the bulb needs to be replaced. The other possibility is that the signal relay needs to be replaced.
A turn signal indicator can be replaced by first removing the burnt out light inside of the glass chassis. The new bulb will then be installed in the same fashion.
If it is making a rapid clicking as compared to the slower intermittent clicking. One of your turn signal bulbs need to be replaced, normally it will only do this when the bulb is not working at all.
Make sure all bulbs are working Replace turn signal flasher
It's the turn signal indicator-located behind the hazard lights.You have to take it out and replace it.
How bout the turn signal indicator switch located in the steering column.
You did not mention if it is a bulb problem or it will not flash. If it blinks faster on one side than the other, then you have a bad bulb. Check them and determine which is not working and replace it. If the indicator stays on steady, then you have a flasher problem. it will not flash
You may have a "ground loop" problem on the left turn signal. Check the ground wire at the front and rear signal bulbs.