99 sable cruise light flashes

A friend has a 99 sable ls. After the cruise is turned on, when the set/accel, resume or coast buttons are pushed the green cruise light just flashes and the cruise never engages. Happens at all speeds. Ideas?

Could be caused by a couple things.
- Vehicle speed sensor
- Brake lamp switch

Does the vehicle have a check engine light?
Have you tried looking at the ECU for a trouble code?

We replaced the brake lamp switch if it's the one attached to the brake pedal. No check engine light. Don't know what the vehicle speed sensor is. I believe each wheel has a speed sensor for the ABS. Is this something separate? Haven't checked ECU codes. Do we have to take it to a mechanic for that? Thanks.