All of the light bulbs work correctly individually. When the running lights are on and the brake pedal is depressed, the lower bulb on the right had side does not illuminate. It does illuminate when the running lights are on, and also when the brake pedal is depressed but, it goes out when both running and brake are activated.
Where do I start looking for a short?
1997 Chevrolet 1500 Express Van with 5.7 Vortec engine and trailering package.

 

Inside the bulb itself. If one filament touches the other, it can cause wierd stuff.

 

Also look at the socket to make sure the bulb seats correctly and that it has no loose contacts and is not corroded.

 

I swapped the left and right side light bulbs and nothing changed. The right side still doesn't work when both the running lights are on and the brake pedal is depressed. I didn't see any corrosion on the bulbs or sockets, and all the wires look OK.
Next idea?

 

When the right side bulb will not come on, move the bulb back and forth in the socket and see if this changes anything. If it does then there the bulb is not seating correctly in the socket.

 

It doesn't seem to be a bulb or socket problem. They are both clean and the bulb is a snug fit with no movement.

 

My guess would be a bad grounding issue somewhere in the line. Check the wires from the bulb down. Also check to see if there is a relay involved.

 

I think I have to connect my trailer lights and see if I have the same problem. I know the trailer lights work, but I didn't check them with both running lights and brakes on. If they work OK, the problem must be in the wires to the back of the van only. If the trailer lights also don't work on the right side when lights are on and brakes applied, I guess I'll have to crawl under the van, find out where the trailer wiring harness begins (it was standard on this vehicle), and check the wiring starting right before the trailer harness.