OK, I've found it. The lousiest design of an item that I've seen with any Fastrack stuff! It's the 6-12035 Lighted Bumpers.
I bought a pair of these, neither one lit for any length of time. After closer examination, it's clear why they're so unreliable. The current for the bulb comes via the rear pot-metal supports that are secured into the center hub. The LED is just stuck into some holes in the support, no positive connection! Small wonder they don't work! Yes, if you wiggle the LED, they'll sometimes light... until the next time. I finally stuck some #32 wire in those holes and ran it down the inside of the supports to make a decent connection. Then we get to the LED protection, no diode, just a resistor. Apparently, you're supposed to buy new ones when the constant reverse voltage in excess of the LED ratings kills the LED. So, I also added a diode. These are new, I shouldn't have to fix them, but by the time I pay to send them back, it's not worth it.
I know this stuff is cheap, but this is beyond cheap!