How would I go about piping in custom animation of a car into the RBD car solver? I have a previous animation done in another software that I'm importing, and I would like to have that animation stop at a certain frame and let the RBD solver take over, with the benefits of the fractures and constraints setup by the RBD car tools. You could normally do this with an RBD configure node and animate the properties on there, but there's too much going on in the RBD car setup for this to work properly.

The RBD Car Transform SOP should help with that - it can take either an animated RBD Car Rig's proxy geo or sim points in the 4th input to drive the RBD Car Rig (or fractured car rig). From there you can animate the animated and active attribs on the RBD Car Rig's proxy geo.
Depending on what your geo looks like, you can recreate the sim points by taking representative geometry for the various RBD Car Rig body parts (body, front left wheel, front right wheel, back left wheel, back right wheel etc), assign them matching names with the ones from the RBD Car Rig, use an Extract Transform SOP to generate the points with the correct attributes (orient, pivot, P and scale) and feed that into the RBD Car Transform SOP.

Ah thank you! I have it properly transforming in SOPs now, but when I put it in the Bullet Solver, the wheels pop out of the chassis on the first simulation frame. When animating purely using the RBD car tools, the rig stays together properly.

If you have an example hip you can share here or through support I can take a look. It's hard to tell what's going on without that unfortunately.

I figured it out! I had "re-align wheels" turned off on the car transform, as well as the "deforming" attribute animating when it shouldn't have been. It works great, albeit some slight popping from the front wheels going inside the collision geo. Thanks for the help!