[kamocat]

Y'know what?
I found the problem, and I feel pretty silly.
Before wiring, I've learned to color-code everything so I don't get it backwards. Most of the connectors on the control system have documentation as to which way they go, right on the case. However, since that's not easy to see in low-light conditions (ie through scratched safety glasses, under a mechanism, in the pit), I color-code.
Since almost everything was labeled, and I was familiar with the control system, I didn't check the documentation before coloring.
I colored the PWM headers on the digital sidecar backwards.

So yes, all is solved. This rookie team had their first drive practice today!