I have a 2000 Venture with an interesting turn signal problem. When you step on the brake pedal, both turn indicators in the dash illuminate. If the right-turn signal is active with the brake pedal depressed, it flashes fast like there's a bad light bulb.I did a visual check on the bulbs and they are all working.

 

Had the exact problem only my was the left turn signal.
I fix mine by looking at the rear tail light assy connector and saw a slightly burned pin and the plastic looked melted. You can replace the connector but I took the assy apart and solder a wire on the ground bus inside the assy and spliced the other end to the ground on the connector wire.I figured I would never have to replaced the rear Tail light assy. If you move the wires around on the connector with the right turn on you might be able to have the lights work correctly. Thats what I did and found the problem. Good Luck