Ok I used the probe I bought and have power to both hot wires on the L& R low beams. I did this by attaching the alligator clip to the body and sticking the probe in the hot wire connector on both sides and then again by connecting the a-clip to what I assume is the ground on the back of the bulb assembly too. Lights up all 4 times on both sides but still no low beam on passenger side.
Well that muddies things a bit. I'm trying to find a picture of what you're dealing with and can't find something thats 100% certain.
This is an H1 bulb yes? Is this what I should be seeing?
Xtec 6000K H1 and H3 Install "HOW TO"
So both sides are powered the same way. Both are grounded (you followed my test above), and you swapped the known GOOD bulb into the bad hole and it doesn't light?
If all the connections are good; then we have a CURRENT ISSUE! The test lamp draws maybe 0.50Amps and will light with very little energy applied. The 55 watt low beam is more like 4.0 amps or so I think. How long has it been this way?
OK - here is DETAILED instructions. Its unlikely that both the +12V supply and the Ground are bad. But one of them is limiting the current. The quick easy way is to run a ground wire from the battery to the lamps ground. DOES it light now? If so, we correct the ground.
If no, we need to find where we lost the energy. Have you tried swapping the relay and the fuses? Or pull them at look for corrosion or burns? It may come down to something in the fuse box. The next step for me would be seeing if the power from the fuse can actually light the 55W light. That would tell us its GOOD up to the fuse. Check or Replace the Relay first and report back.
Hint: 13V X 4.2A = 55Watts so the ground and the supply need to be able to pass at least 8 amps to initially light the light as it begins to glow the current drops.