So I know to get the wheels going using the RBD car rig and the bullet solver you have to override motor_target, goal_twist_axis, and goal_up_axis in the constraints tab of the bullet solver (SOP level). That does the trick for speed and steer, but the brakes still aren't working. What do I need to override for aggressive braking? (I'm using two different car rigs--one for a semi, the other for a trailer, both packed and merged.)

In case anyone else in the world ever needs this, I found the answer--"targetw" has to be included in the constraint attributes to be overridden.