The low beam suddenly went out. The high beams work fine?

If both low beam bulbs and fuses have been checked and found to be in working order, the next possible culprit could indeed be the relay. The relay controls the electrical current that flows to the headlights, including the low beam. A malfunctioning relay can cause the low beams to fail while the high beams continue to work.

To confirm if the relay is the problem, you can try swapping it with a similar relay in the fuse box, such as one for the high beams or another electrical component with the same specifications. If the low beams start working after the swap, then the relay is likely faulty and should be replaced.

As for accessing the switch, it depends on the make and model of your vehicle. In many cases, the headlight switch is located on the steering column or dashboard. You may need to remove some trim panels or covers to gain access to the switch assembly. Consult your vehicle's manual or search for specific instructions online for your particular make and model. If you're not comfortable with accessing the switch yourself, it's recommended to consult a professional mechanic or an automotive electrician to assist you with the diagnosis and repair.

So just the low beam driver side is inop?

It could be a bad fuse, relay or the bad body ECU. here is a guide to help you test for power so you can see where it loses it at, and the headlight wiring diagrams in the images below.

https://www.2carpros.com/articles/how-to-use-a-test-light-circuit-tester

Check out the images (below). Let us know what happens and please upload pictures or videos of the problem so we can see what's going on.