Ok, so I'm trying to get this Pathfinder up and running and the latest problem is with the headlights: the left lamp has a non-functional low-beam, the right lamp has a non-functional high beam. (It's very weird to switch from low to high beams and see the beam of light jump from one side of the car to the other.) I put in new bulbs but that didn't solve the problem. So...

a) I don't see any bad fuses -- and a bad fuse would cause the whole lamp to go out, right?
b) I tried switching around the relays for the lamps, swapping them across and triangulating with a third relay swap... still no difference. So it doesn't seem to be a relay problem.

My next go-to would be to check the switch, but... I don't know how to do that beyond heading down to a junk yard, etc. etc., and that seems like a lot of effort to go through when I don't KNOW that's the problem.

Also, when I tried to remove the old bulb from the right side, it had kind of fused to the plug, which suggests something weird going on somewhere in the electrical system -- like it got too much electricity, or the gauge of the wire going to it was to small, or something (except the wires all seem to be original).

Any recommendations for the next step here, or is this where I throw in the towel and go to a "real mechanic"?

Oh. Also, the daytime running lights seem fine on both sides. So... on the left we've got: yes daytime running lights, no low beam, yes high beam; on the right, yes daytime running lights, yes low beam, no high beam.

The headlamp stalk/switch sounds like the culprit. This is fairly easy to test with a multimeter and the Factory Service Manual, available here: https://www.nicoclub.com/nissan-service-manuals.

Drill down to Pathfinder > 2001.5 > Electrical System. Since 2001.5 is a conversion year, you'll need to see the note about VIN cutover to know if you have the "Previous" or "New" Smart Entrance Control Unit, then look at the Headlamp section depending on US or Canada. Since you have DRLs, guessing Canada?

Anyway, you'll find system descriptions, the schematic, and a wiring diagram for the entire circuit. Once you remove the steering column cover, you'll have access to the wiring harness at the stalk and can use voltmeter to check for voltages before and after the switch. The wiring diagram indicates the harness shape, pin locations, and wire color (see the General Information chapter for the wiring colors, but typically they're pretty straightforward except L = Blue, B= Black, BR = Brown). You should also confirm all grounds are good.

You can do this at the headlight/bulb harnesses if you'd like, but at the stalk is the best place. If the stalk is deemed bad, junkyard is one route, used ones on ebay, or new ones on RockAuto and probably Amazon. Just make sure you match it to whatever features you have, like fog lights and Auto headlights.

The Canadian models all used a "Daytime Light Control Unit" that dim-bulbed the headlights by putting them in series for DRL operation or parallel for normal operation. The DLCU's are notorious for cracked solder joints, so that would be the first thing I'd check. Very often just retouching all the solder will get them working right again.

The burnt connector at the bulb is a common situation, it's from crummy terminals (usually the ground pin) and not from too much voltage. You can find plenty of replacements on eBay and Amazon, the aftermarket makes them because they fail pretty regularly.

Thanks a ton! I'll work on this and let you know if it fixes the problem.