Oops. I woke up this morning with my cranial OBD working better and remembered that the reason I'd had the cluster out was from trying to diagnose an no charge condition.

I tried a cluster from an 87 because I had one, and that solved the problem. The manual shows a different cruise system on the 87 however, and different wiring to the speedometer, so I think therein lies my problem.

Looks like I'll have to either repair or replace the appropriate cluster if I want both charging and cruise.