Headlights dim signifigantly when power windows roll up or down

Both lo & hi are out ? The headlamps are controlled by the BCM - body control module . HDLP Lo & HDLP Hi relay's are turned on (energized) by the BCM . Headlamp switch is a input to the BCM . Does the charging (alternator) system charge the battery ? Battery light lit on the instrument cluster ? Your best bet , Take it to a qualified repair shop.

Here'sa link to where your fuses are. Replace Fuse 2007 2013 GMC Yukon 2008 GMC Yukon Denali 6 2L V8

It could be. But first check to make sure the small wire coming from your ground cable to your fender is secure. If this cable is not attatched or loose, the headlights will flicker.

sound like battery has weak or battery terminals and post need cleaning. have battery charged and checked, if battery good. alternator not charging battery. if battery over 5 years old more likely battery is bad.if battery too weak head lights wont turn on battery dead this time of year if battery old. cold weather will finish it out.

Yes this can be a normal condition caused by load being applied to the headlights and that lowers the voltage, the lites dim, then the load is removed and the lights brighten. Typical loads that cause this are AC climate control related. Have the Alternator load tested to confirm there is no problem in the charging system. Also have the battery tested, a battery that is starting to fail can cause this problem as well, a failing battery takes a huge amount of charging system power to keep it up, so that makes the lighting see higher spikes in voltage

You have a wire grounding somwhere, Look for any cut wires or frayed wires,

Headlights dimming and brightening indicate a change in output voltage from alternator, not a problem as long as its within certain guidelines, usually 12.4volts - 13.6 volts (approx) . Overcharging alternators will produce a rotten egg odour from battery as it dries out. Put a voltage metre on the enging at idle and check upper range .

Things like this are usually from a failing ignition switch for Trailblazers and Envoys. Hope this helps..

You may need to replace the auto headlight switch.

Just replace the bulbs,they don't last that long.The trick is remove the headlights,there is (2) pins on top of headlights.
Remove the pins and pull the headlights out the way.The light sockets are right below!