Well...that NOS speedometer is expensive. But at least it will work. Removing mine left a pile of tiny springs and a broken needle. Not the best thing ever, I'm afraid.

Now: I have an RTE solid-state relay en route, but can somebody tell me what the two wires could possibly be doing here?

Black runs from right prong and grounds underneath the radio.
Red runs from column to one of the mounting bolts of the radio.