Speed sensor is easy.

2 wires come out of the sensor on the trans. They run to the cluster. They travel across the lower engine harness, into the engine bay (not engine) harness, around the front of the car to the super multiple junction, and into the cluster harness. The gauge gets the speed from the sensor directly. I think the wires are yellow/black and yellow/blue, so check them where the lower harness connects by the relay box.

The ECU is actually driven by the cluster. That wire is yellow/green. If this wire is missing, the ECU won't know how fast you're going but the cluster will still work. That wire comes down the body harness to the F1 dash plug, and down to the ECU.