How to replace the headlights on my 2003 GMC envoy.

dis-connect both battery cables from the battery.
Touch them together and re-connect them to the battery.
remember disconnect both, don't be lazy and only disconnect one.

Drivers side- stick your hand behind the headlight and remove the rubber cover, and twist out the light.

Passenger side- remove the air cleaner top, and move the air cleaner out of the way. You now have complete access to this side to remove the rubber boot covering the bulb.

Relay in fuse box under hood on driver side wheel well labeled HDM, #46 on 2004 Envoy XUV. Swap with relay #45 labeled "Fan" to test. That was my problem.

Good Luck,
Sam

there is an ignition fuse in the main fuse block under the hood, if thats not the problem then check the fuesable link,and netrual safty switch!!!

test and check the headlight switch -wiring , connections and sockets for power

Both low beam head lights are burnt out? blown fuse.

The head light can be the problem.have the tech,check,or replace it.

I would start by checking the headlight relay and headlight fuse.  You also may wanna check to make sure the bulbs aren't blown out.  If those parts check out to be fine and in working order, you may need to replace the turn signal assembly in the steering column.  This part controls the hi/low beams.  This may sound crazy but, when GM wired their older trucks, they wired the headlights to go from the battery, to the hi/low beam switch, then to the headlight switch (to the left of the steering wheel) then out to the light bulbs.  You can kind of test to see if this part is good by making a fist and (with your headlights on) gently hitting the plastic cover for your steering column that is right behind your actual steering wheel.  This could cause your headlights to flicker on and off.  It's not 100% guarenteed that this method will work, but it may.  
Any questions, just ask!

open the hood.
there are 2 small steel "tabs" which you need to pull straight up.
once these are in the up position, pull headlight assembly forward.
remove plug connector to headlight, prying the wiring tab with a screwdriver.
rubber cover can be pulled off exposing the back of the headlamp bulb.

The headlights on the envoy are the easiest parts of the car to repair. You just pull the two tabs at the top of the headlights, screw out the bulb containers and you are good to go