First question here, and don't work on electronics often, so tips are welcome. This might have a very simple solution.
I tried to use the schematic editor to the best of my knowledge to recreate an aftermarket headlight on my car that has stopped working (U1 and Lamp 3 are inside the headlight). The contents of the U1 box is my question. The 100 ohms on the lights were the default of the editor and is not accurate.
The ground wire might go into U1, and Lamp 3 might pick up the ground from U1, but I drew it outside for simplicity.
Lamp 3 is a LED strip. When the headlight switch is in the "OFF" position, SW2 is open, SW1 is closed, and Lamp 1 (the daytime running lights) is on, and Lamp 3 should be on. Right now the LED is not on because of a failed part in U1. When the headlight switch is "ON", SW1 is open, SW2 is closed and Lamp 2 is on (the actual headlight). Lamp 3 is currently on and working as it should.
I need to replace U1, which is enshrouded in heat-shrink (1/2" x 3/4" rectangle) and located in an unreachable area of the housing. I have access to all the input and output wires, so I can fit a replacement once it is built without damaging the light.
Main question: What parts are in U1 that will allow two power sources to feed Lamp 3 without the two power sources touching each other and back-feeding? For instance, if U1 was just a wire-nut, Lamp 1 and 2 would both be on if either SW1 and SW2 were on. To my knowledge, SW1 and SW2 are never on at the same time. Lamp 3 is an LED strip, and I would expect it to draw < 5A. The entire system is 12-14VDC (automotive system). Temperature range is extreme - probably from 0-200 deg F because of the proximity to a high-wattage halogen bulb, car engine, sun, etc.