Try this. You will need a multimeter, set it to measure resistance.
Locate the light blue/black and dark green/orange wires on the servo. They should be right next to each other. These are the wires that connect to the steering wheel switches. Unplug the connector from the servo.
Connect the multimeter to measure resistance between the light blue/black and dark green/orange wires.
Now have a helper push the on/off switch on the steering wheel. The meter should read a very low resistance, under say 15 ohms.
Now have a helper push the resume button. The meter should read around 2200 ohms. Push the set/accel button, the meter should read around 680 ohms. Push the coast button, the meter should read around 120 ohms.
The dark green/orange wire is on position 6. The light blue/black wire is on position 5. For reference, position 2 is empty and so is position 8.
If the above test passes then check the following:
Check position 7 (white/pink) with a test light for power with the key on. It should have power.
Also check position 4 (light green) with a test light for a change when the brake pedal is pushed. This is connected to the other brake switch (there are two). It's supposed to have 12V only when the brake pedal is pushed.
Also check position 9 (orange) with a test light. It's supposed to have 12V only when the brake pedal IS NOT pushed.
Finally, connect your test light to the battery + terminal and apply it to position 10 (black) to check for a good ground. If it doesn't light then the ground is bad.
If all of these check out then that only leaves the VSS as the possible culprit (besides the servo itself).
Jack up the rear of the car so the rear wheels can turn freely, connect your multimeter between terminal 3 (gray/black) and ground, set to AC volts, and verify that the voltage increases as the rear wheel speed increases. You should probably get a good reading with the rear wheels going no more than 10MPH.
If this checks out then the servo itself is bad.
