Omlu

I own a 2010 RX. A couple nights ago we noticed one of the headlights was out - the passenger side to be specific. The HI-beam still worked but the Low-Beam did not. We purchased new headlights and replaced both sides but the passenger Low-Beam still didn't come on. I tried switching the bulbs to make sure it wasn't a bulb. I also tried switching out the Low-Beam fuses and it didn't fix it. For all cases, the HI-beam will still turn on.

Any ideas?

Non

First thing to check, if replacing the bulb did not fix it, is the chaise ground connector. One of the wires coming from the bulb is usually connected to the body of your car and the screw corrodes sometimes.
Using a voltmeter would be very helpful here.

SnakeDocto

I have a 2013, and driver side xenon light was out, so I purchased a replacement, put it in, and it still didn't work. Swapped the ballasts from each side, now both lights don't work... Swapped the bulbs with the ballasts still on opposite sides of their original location, nothing. Swapped everything back to before (new bulb driver side). The 10A Bi Xenon fuse is still good. Swapped it with another 10A fuse just to make sure, and it's good. I suppose I can check with the voltmeter, but it's weird that both sides now don't work.