I know, right? You could get a smashed up lower bedside panel repaired for that kind of money!
Holy crap Luis, something isn’t right here - front LED light bar requires new taillight assembly? Have you tried disconnecting the light bar and returning it to stock?
Looks like a worst case estimate, the kind they write up when they don't really want the job. I doubt the tail lights need anything unless you were tapping into wires back there for some unknown reason. But, improperly wired could easily take out the BCM. But, you could pay the $400 for the diag, then get a better estimate.
Which LED light bar were you wiring in, got a link to the manufacturer's page for it? How were you planning to control the light bar? How did you wire them in?
FIRST - DISCONNECT THE LIGHTBAR
It's Clearly shorting the circuit or creating an issue.
Then post how you wired it and how many watts it is.
The BCM separates the circuits Brake/Park/Turn/Reverse - and splits the circuits in half by 2 fuses.
For all the lights on the (Rear) to have an issue, the BCM is killing power (FET) Protection to the circuits.