on my 2005 trailblazer the headlights will turn themselves on.

Do you have lights that turn on automatically after dark? The switch may be defective. If not you may have a short in the switch. The only way lights will turn on is if there is power applied to the system. The switch is where the break is in the electrical system. If you can, feel the switch after the lights are on and see if you can feel heat from an electrical short. Find out quickly before the fire department is called to put out the fire.

solid state relay i do beleive its 46 is failing just had it happen to me the relay is in the fuse box

Well I just thought of something. Is this part of the GM recall on ignition switches? Check it out as to what the issues were on the recall. If the symptoms are the same, talk to GM, not a dealer, but to GM about the possible switch being defective.

The same problem happened exactly to my car Trailblazers 2005! When I went to garage the Mechanical said to me bullshit things such as he has to chick all car Electrical system to find what kills the battery and it's gonna cost me $1250! Then I asked a friend he has a garage too, he asked me to chick the box fuse and replace the relay that feeds the headlamps which was number 45. So I changed the relay number 45 with new one costs me $31 and it worked with me. Be careful, You don't need to change the battery, it's just the relay that feeds the headlamps which kills the bettrery while the vehicle has been locked!

I too have a 2005 Trailblazer with the same issue... .just went out and bought a relay and it fixed the problem.... $37.00

Thanks for posting this. I had the same issue with this on my sons 2004 Trailblazer. After reading the comments I temporarily switched relay 45 (fan relay) and relay 46 (headlight relay). The headlights went off and functioned as the should. We replaced the defective relay and everything works properly now

Hussain thanks so much for the info! My 2005 Blazer is doing the same thing! Lights coming on and off all day and night, leaving me with a dead battery everyday...I going out to get that switch Asap!

Same thing is going on with my 2003. I also have the 5.3 v8. Thanks for posting this, it will save me some problem chasing!

the same thing is happening to my 2003 trailblazer... could it be the same problem???

Had the same problem with my 04 trailblazer. Apparently there is a built in failure warning for the fan clutch relay. When it fails, the head light relay will turn the lights on when the vehicle is off to tell you the clutch relay has failed. Changing the clutch relay fixes the problem.

Same issue, just bought a battery needlessly. 2005 5.3 V8 Envoy

Just bought a new battery. Now noticed headlights turn on when car is locked. Now going to buy relay switch after reading posts. 2004 Chevy Trailblazer. Thank you all. Never would have figured it out.

Thanks to all who correctly answered this question. I had the same issue with my 05 EXT and was dreading the idea of paying a fortune to chase down electrical gremlins on an 18 year old TB with close to 224,000 miles on it. Bought two of the "premium" relays at Advance for about $35 each before discounts. The non-premium version was $5 cheaper. According to an employee, the difference is a 1 yr vs Lifetime warranty. Decided to replace both #45 and #46 relays as they're probably the same age. Worked like a charm.