Sounds like you may have had a ground issue.
Or a connection issue at plug.
If a magnet intermittently doesn't work it will say check plug and likewise if there's a bad ground.
When the integral brake controller is set up properly when you hit the brakes you should slightly feel the back of tv pull down a bit.
Btw this is all in your gm owners manual.
Read 4 thru 7
Goshen is my playground.
The last man standing also has a sharpie