How can I replace high level brake light lamps?

you have 3 brake lights one your car one in each rear lamp clusters and the high level lamp in the tailgate what are you trying to replace?

You do have a high level stop lamp. Don't believe I've ever heard of a high level or 3rd tail lamp? Start the vehicle, in park, parking brake set. Turn on your headlamps, look at rear tail lamps, anything out?

Your lower brake lamps are wired through the multifunction switch. The high-mount brake lamp isn't wired that way.

At the lower rear, the turn and stop use the same bulb filament, just not at the same time.

I'd have to check circuits to and from multifunction switch. Sometimes you can get under the dash, find wiring harness to/from steering column, If you can find the proper color wire for lower brake lamps, see if voltage on that wire with brake activated, if no voltage, the problem could be the wiring in steering column or multifunction switch.

High level brake light is connected , in parallel with, all other brake/stop lights.. It is not on a separate fuse, so, check, or replace the other stop lights.
Or could be a bad ground on brake lights, but not on high level light.

check the lt blue wire at upper lamp,(under trunk lid trim panel) for 12 volts when brake is applied. if no voltage,check same color wire at brake lamp switch for voltage.(by brake pedal).if no voltage replace stop lamp switch asm. if voltage is detected at rear by lamp then replace CHMSL asm.(center high mount stop lamp). this lamp is made of many LED's,if some work and some don't you will need to replace lamp asm. can be repaired by a trained GM tech, possibly. there is no seperate fuse or relay for this lamp,only a stop lamp fuse.

The high mount brake light has its own fuse, so check that first. The high mount bulb is generally a stabdard type (there may be more than one bulb) or you can replace with an LED. As for the left rear bulbs not working when depressing brake pedal, check the bulbs again for any spring that may be broken. If the bulbs are still good, make sure there is sufficient contact with the socket. If still not working, have someone check it out further.

If the high level brake lamp works and the other 2 lamps have good bulbs, then there is a power loss, most propably with the installation. You should go to an auto electrician to see where the break is.
If someone has recently done any work on the electrics, you should recheck. It is not a seperate fusable power supply system or brake switch for the high level brake lamp, it will take its power from the fused system that the other 2 have.
Hope this helps

i beleive the cover snaps on if not you may need to remove door trim

not sure which top tail light you mean? do you wish to replace bulb or actual lamp on the rear screen? this is the high level brake light ! OR do you wish to replace bulb in one of the rear tail lamps offside/nearside ? which ever ? the h/level brake light bulb is accessable for bulb from the rear panel on hatch behind lamp bulb is 16 watt capless push in type