I wouldn't be so quick to blame the car. By the sounds of the supplemental braking you installed it had to be wired to the car. That's were my suspicions would be. Not wired correctly or it's causing a fault in the cars computers.
We have the Brake Buddy and have had zero issues with the last two jeeps, including our Jeep Cherokee Trailhawk which we've had going on two years. It doesn't wire into the cars wiring. Too many electrical systems in these new cars IMHO and I wasn't about to start splicing wires - and that included the brake lights ( we have wireless magnetic ones).
So that is what I would suspect, along with there is a specific way they need to be put into nuetral. If you fail to follow them correctly it won't work.
Go here - a lot of information on how they are installed. Specifically noted it won't work on all vehicles along with other warnings. Are you sure this installation was done correctly? If it wasn't the dealer is going to point a finger at this.
Just a snipet
CAUTION
Although not recommended, if single terminal
diodes are used they must be installed in each of the wiring circuits. Otherwise, InvisiBrake may not function properly. Non-warranty electrical damage may also occur to a vehicle with an unprotected circuit.http://roadmasterinc.com/pdf/85-4649-33.pdf