2002 Ford Explorer tail light won't work.

Have you checked for power to the socket with a test lights? The socket itself may be bad and not making a connection. THe brake and signal light work from a different circuit and element. As far as under the hood, check the fuses, but I don't think there are seperate circuits for the left and right driving light.

I have not checked with test lights, but I did do a voltmeter test on the drivers and passengers side. I have 14v on the passenger side and 2 v on the drivers side. I switched the socket to the passenger side and it worked fine. So what do I do now? I did check all the fuses under the hood...I didn't see any issues.

Funny I have the exact same problem. My Rear drivers side running light will not come on with the headlight switch turned on. The brake lights work fine, turn signals work fine regardless of whether headlights/running lights are on. I have changed bulbs and sockets and no change. Also, there is a common ground and two supplys to the socket. The supply voltage to the brake / turn signal is 11.64 volts to ground pin (with no alternator running) and the supply for the running light is a volt lower approximately 10.49v. This looks like a loose or broken wire somewhere. I am thinking difference of potential but no current flow which is possible. Was thinking of splicing in the trailer hot for running light since the common ground cannot be the issue?

Yeah I was trying to avoid splicing anything, but it looks like I'm going to have to back track the wires to see where the issue is....sounds not fun.

I never did find the issue, I back tracked the wires to behind the drivers seat and then gave up. I spliced in a wire form the brown wire on the passenger side taillight, and feed a new line to the brown on the drivers side tail light and spliced it in. Not what I wanted to do...but I'm not riping my car apart.