I have tried to install LED bulbs in my brake light fixtures. They don't work except when the brake is actuated. So, I tested the circuits and found the issue but, not the cause. When I turn my lights on I only get about 10 volts at the running lights. The LED bulbs won't work with just 10 volts. When I activate the brakes I get 12 volts for the brake lights.

Any ideas why I only get 10 volts at the tail lights (there's three).

 

My first guess is corrosion on the connections.

 

As Texas suggests, there is probably a poor connection somewhere. Just go back through the wiring, checking voltage at each connector, especially grounds.

 

I agree...need to become Sherlock Holmes...

 

I put some aftermarket 3157 style LED bulbs in my Ford Bronco and later had an issue where I was only reading partial voltage at one of the bulb sockets. Was getting half the light output on both sides and found out one of the bulbs was bad causing partial voltages on the running light side. Worth a shot.