2005 GMC Sierra 2500HD, left side low and high beams out, new bulbs, fuses good. No power at the sockets. Could this be the high/low switch on the column?

sounds like partial short in headlight switch

check low beam fuse when switched on if it has voltage registering across the fuse ,if there is voltage across the low beam RHS fuse then it is possibly a pinched wire from the headlight running back towards inside of the car

Check for power at the socket. If you have power there bulb is not making good contact.

id just replace the bulb, socket,and your sure you checked all fuses

seems you have several different problems. High beam working and no low beams=dimmer switch is bad. Reverse lamp not working=bulb socket bad,wiring broke,bad ground at back
Brake light on drivers side stuck on=brake light switch

use dc voltage meter and see if you have power in the sockets and if the fuse is good and relay , also check to make sure you haven't lost the ground.

You need a new headlight switch my friend.

a bulb on one side and new light stalk switch

i had same p w 1996 gmc chgd bulb chgd switch checked wires in steering coloumfinmally went under dash where your left boot would be and wiggled a 3quarter by half inch relay and done it lite up all day of jackin 2sec fix

Power flows from the battery to the main fuse. Then it flows to the control coil and contact side of the headlight relay. When you turn on the headlights, the headlight switch provides ground to the control coil in the headlight relay and that operates the contacts. The contacts allow power to flow to the right and left hand headlight fuses. Power flows to the headlights. The headlight switch is what provides ground to the high and low beams on the headlights.

To diagnose, remove the headlight relay. Using a voltmeter, check for battery voltage on terminals 3 & 4 in the socket with the key in the RUN position. Then turn on the low beams. Test for good ground on terminal 1 in the socket. If you're not getting ground, then you've got a problem with either a bad headlight combo switch or a bad ground. If you get good ground on terminal 1, reinstall the relay, turn on the low beams and test for power in the headlight sockets. If you get battery voltage, that confirms you've got good fuses and wiring up to that point. Next, check for good low beam ground on the red/green wire in the bulb socket. If you're not getting good ground, it's either a bad combo switch or bad ground from the combo switch. Then turn on the high beams and check for good ground on the red/yellow wire in the bulb sockets.

It wouldn't hurt to clean the ID ground on the left side of the instrument panel, but based on everything you've said so far, this sounds like a bad combo switch