96 XJ here. About a year ago I installed an aftermarket 55/100 headlight wiring harness from SMC Truck...



...and just now both my low beams went out. High beams are still working though.

I don't think it's any grounding problem at the headlights as each side is ground individually and, like before, the highs still work.

Where should I begin testing?

I assume the fuse is still good, and that maybe the problem is at one of the two relays. Both relays are a 1" cubed black box with writing...

SHORI
S11-1A-C1-12VDC
30A/12VDC
COIL:12VDC

Is there a way to test the relay maybe? I have a multimeter but not enough knowledge to just jump into this.

Thanks

High probability it is a relay. BUT, not sure if that configuration has a relay per side or if one relay is for high and the other low.

When you look at the relay, it will give a description of the actuating coil and the open/closed connections.

Apply 12 volts to the actuating coil and see if the relay opens and closes as it should. Generally the relays are in a normal open state and close when 12 volts is applied to the coil.

If the relays are okay, the next step is to return to your headlight switch to see if it is functioning properly. Pep Boys used to have a relay tester.

The headlight switch is prone to corrosion and dust that will cause problems. As you have lost low but have high, it is possible the low connections in the headlight switch (or connector) are at fault.