I just bought a 2002 trailblazer. All lights interior and exterior constantly flicker dim then bright. Battery is good, I thought maybe voltage regulator but the alt was bought 9 months ago and I also just had that tested and was told it was good. Any suggestions would be appreciated

 

Sounds like a bad dimmer circuit. Grab a headlight switch assembly from a junk yard and swap it out and see if anything changes.

 

Known issue on the GMT360/370 platform vehicle. It can sometimes be an issue with the alternator, but more often than not, it is an issue with the ground circuit(s); typically grounds 102 & SP201. Many have fixed this issue by adding grounds to the system from the battery negative to the frame as well as to the engine block. It is possible to check the grounds to both of these locations, but the problem seems to reoccur for a lot of folks.

Ground 102 is located on the inside of the driver side fender near where the fender meets the firewall. Ground SP201 is located under the center console. SP201 location on Youtube for a visual and some good information. There are even more videos on Youtube concerning electrical issues on these vehicles.