I just bought a 2005 Envoy truck a couple months
ago and my headlights went out I thought it was
the bulbs so I bought two brand new headlights
and put them in but sadly my lights still didn't
come on only my right passenger side high beam
worked then later after that my interior lights all
went out. I was told it was something electrical but
I'm not sure how this problem can be solved, can
Note the diagram. You should have checked voltage and ground for headlamps before replacing anything. You can use a test light to check both. You see the headlamp fuses, hi and lo beam, with headlamp switch on, use a testlight and voltage on those fuses, both hi and lo. You can see relays in wiring diagram. There are trouble codes for headlamp problems, the ones I saw are B type codes. Around here some of the national brand auto stores will check codes for free. I don't know if they will check manufacturer specific codes or just PO codes?
There is a recall for the issue of low beams not working. Swap these 2 relays, under the hood shown in the picture below. You'll have your low beams back. Just buy a new relay, and keep a spare. For your interior lights, check fuses #4 and #40 under the rear fuse box under the rear drivers side seat.
I ran into the same problem with head lights going out in my 2002 gmc envoy and my fuse panel under the hood was loose so I tightened the bolts and headlights worked so it was loose connection.