The basic circuit for a car lamp is for a single wire to carry +12v to it, and the second wire goes from the lamp to a local earth point or an earth brought in to the plug from somewhere else in the car. The whole body is earthed and on a car that just means the battery negative (-) is connected to the metal car body, also called the ground. That way most items will only need a single wire to power them and they can "earth" locally to save costs in manufacture.
Some LED lights however are fed from a "driver" circuit, which could be the black box. A driver unit sends a stream of pulses to the LED and not a straight 12v supply. The driver box itself however, still gets fed a normal 12v power feed over 2 wires or one and a locally earthed one.
If you have a mobile phone with a camera handy, try filming the working LED on the good side, then watch the screen of the phone or play the video back, if the light seems to be flickering on and off rapidly, its a driver system you have so the black box will be that item and it may not have a 12v supply coming into it, hence not working. Cameras show this happening due to their refresh or scanning rate which the naked eye cannot see. With my dashcam I can see which traffic lights or car lights are LED or conventional filament lamps by whether they flicker rapidly or not, on the camera screen..
Your LED tail unit could have blown the driver and so neither work. Have you tried plugging the new LED you bought, into the side that works, this would show you the light unit and the driver are dead so you would need to prove there is, or isnt, a 12v feed to the driver box when the lighting is on and then go from there. If there is, you need a driver unit too. If not, you need to trace why there is no supply. I'd be surprised if there was no fuse, there always is to every item in a car for safety reasons. Unfused wiring or components can cause wiring fires rapidly so there must be a fuse somewhere.
Hi this is driving me nuts where is this bulb?
