Truck Repair: electrical on 85 gmc pickup, turn signal switch, gmc 4x4

Question
I have a 1985 GMC 4x4 1/2 ton.  The passenger's side rear brake light/right turn signal doesn't work.  On the same bulb is the parking light which works.  I have changed bulbs(both new ones and different styles); I have check the wires for any noticable breaks and found none. I made sure the lights are grounded.  I replaced the the socket with a brand new one.  I have tested the with a meter and the driver's side reads the same as the passengers but the drivers side works fine.  What could be wrong?  What are a few things I should try next?

Answer
Hi Jay,
You said you checked them with a meter, but did you check with the light on, like the turn signal on, to see if there is voltage?
It is remotely possible that the problem is in the turn signal switch in the steering column. Both the signal, and the brake light power must go through that switch.

Just because the parking light works doesn't mean the socket is well grounded. It could be feeding back through the turn filiment of the bulb, and would be dim, and appear to work right.
When checking with a meter, or a test light, connect the ground of the testing device to a good ground and see what happens, and then connect that ground lead to the brass socket of the light you are testing, to see if it performs the same there. Lack of ground to that socket is what it sounds like, and would sure be a easier fix than the signal switch, and you surely want to be sure which it is.

Something to try, would be a little jumper wire from a good clean frame ground, and hold it against the side of the bulb socket.

Good luck,
Van