2005 chevy trailblazer - no cruise control and brake lights only work when key is turned off

brake lights in rear of car? or brake lamps in dash? ??cluster.ABS? I can't see where your eyes are looking.
if rear lamps fail IN ANY WAY, that kills dead Cruise by design. so fix rear brake lamps first.
so fix the brakes in your other post.
and your engine is
4.2Liter, inline 6, 2 cams, 1 cmp and has VVT
and VVT is failing I BET!
I'd fix the engine first, for sure VVT failing
is never good and for sure if cam is now out of time by 25degrees.(VVT stuck hard max , jammed)
tell here eyes are looking and win

brake lamps will not TURN off.
brake lamps stuck on all the time.?
they never need key on to use brake lamps.
the brake fuse is hot all the time. is why.
and is a safety rule this ,just like head lamps.work key off.
if stuck on rear brake lamps
pull the switch, plug at pedal switch
stuck on NOW????????????????????

There are several possible reasons why the 3rd brake light on your 2005 Chevy Malibu works only with the key in the accessory position and not when the car is running.

1. The brake light switch: The brake light switch is responsible for turning on the brake lights when the pedal is depressed. If the switch is malfunctioning, it may not be sending the signal to the brake light when the key is in the "on" or "start" position.
2. Electrical issue: An electrical issue with the wiring or a connection in the brake light circuit could be preventing the brake light from turning on.
3. The Body Control Module (BCM): The BCM controls various electrical functions in the car, including the brake lights. A malfunctioning BCM could prevent the brake light from turning on.
4. The Brake Light Relay: The relay could be defective and not allowing power to reach the brake light when the key is in the "on" or "start" position.
5. The code PO573 that is coming up is related to the brake light switch, it could be a symptom of the problem.
6. The cruise control not working can also be related to a problem with the brake light switch.

Your owners manual should show you where the fuses are located. If you don't have an owners manual, you can go to
my.gm.com, type in your year--make--model and hopefully read your owners manual for free.
Do the brake lamps work ok?
I found these fuses, not only check fuses, check for voltage on fuse circuit, use a test light. Some fuses are hot all the time, some go hot with the key on.
Any codes for the cruise system?

First you need to have EBCM - electronic brake control module check for DTC'S - diagnostic trouble code's . If the light is on there is a code stored . I think most part stores can do this and for free . After you find out what code is stored , looking up that code in service repair information for diagnostic trouble shooting chart would be next step . There are a few codes too.
DTC C0035 or C0040
DTC C0045 or C0050
DTC C0110
DTC C0131
DTC C0161
DTC C0186
DTC C0196
DTC C0201
DTC C0240
DTC C0241 or P0856
DTC C0244 or P1689
DTC C0245
DTC C0283
DTC C0287
DTC C0290 or C0292
DTC C0455
DTC C0550
DTC C0558

Find the code do the diagnostic's , fix the problem , wiring issue's , a sensor , bad module ,what ever it is . The light will go out then . Here is a sample of code discription .
• DTC C0035 Left Front Wheel Speed Sensor Circuit
• DTC C0040 Right Front Wheel Speed Sensor Circuit

My: first guess is if you are.using the orginal key? Those vech. Have a.vat.

check cruise control fuse and check the brake lights see if brake light working because the brake light switch will deactivate cruise control when brakes is applied.if brake lights dont work either fuse blowed or switch bad.if all is good have vechicle code scan because cruise control control by PCM and vechicle speed sensor and thottle actuator control motor.

i had the same problem, i had a mechanic tell me it could either be a switch or my connection in the back brake light. i was told to drive my trailblazer down the road a little bit. turn on my cruise control and then push on my brake. if my cruise control light went off after pushing the brake then it was my connection in my brake. if it didnt it was my switch. i ended up having to replace the whole light panel in the back on both sides, it was the connection that was bad. it was actually an inexpensive fix. hope this helps.

Cruise control deactivates once you press the brake pedal. The brake switch us an input to the cruise control module. So it is possible that your brake switch went bad keping the lights on and making the cruise control module think you are applying the brakes when in fact you are not. Replace the brake light switch and check operation.

If the brake light it on it is generally because you are low on brake fluid.

run the fault codes
check that the camshafts are turning ( broken belt /chain)
check for failed crank/cam position sensor/s