The issue in the end was that the speedometer itself, the one with the mileage counter, that was fitted was meant for a late model cluster, rather than the early model needed for my 1990. Essentially, the cable from the ECU that connects to the speedo had nothing to connect to or read, hence no speed reading and resulting in the HICAS light. The continuity from the ECU to the speedo plug came back fine, no wire degradation. From what I remember, I think the actual speed sensor wire was green and yellow if that helps.
Early model speedos have this extra wiring bit: