2008 trailblazer. no brakes at all! not 3rd light either. fuses were good. now what?

The 3rd brake light and the tow side lights are on different fuses, have you verified that they are both good. The other part that can cause this is the multifunction switch (AKA turn signal switch).

Bulb or contact..wiring cuddabin touchd

The tail light fuse is under the rear seat on the drivers side.

Usually not. Most vehicles I've worked on have had the brake lights and 3rd operate on 2 different systems, likely as a safety measure.

Sounds like the new system you installed is piggy backed off of the main brake circuit and popped the fuse.

its a fuse,or that circuit has a short .in the old days your dash lights would be out also.but trailblazer electrical defies logic for me.there is a circuit board for taillights.but if there all out look for burned out fuse.dont forget to check out side ones also.i seem to remember a fuse block under back seat but mine was an early one.one i had, had every bulb was burned out never knew why,wife traded it in with 13000 miles on it.it liked the dealer

The third brake light circuit bypasses the turn signal switch.
I would check the turn signal switch for power in and out with the brake pedal depressed.

check the brake light switch located on the top of the brake paddle also check the butten on it if its stuck in.

u either got a ground wire loose or a fuse blowed check under the dash and the hood there should be two different fuse boxes check both of them

That auto has a recall for the listed trouble