I have a collection of the circuit boards that mount under the dash, from junk yards, and changed them to fix it a couple of times.
If it is an auto, make sure the vacuum dump valve on the brake pedal is sealing closed when the brake is off. Not sure if there is a clutch switch on a stick car or not. But check to see if there is.
Make sure the speedometer pickup is plugged in, and there are not other vacuum leaks.
The last think I have replaced is the steering wheel switchs. It might pay to clean the rotary collector on the steering wheel.
if the car has an airbag and the air bag light is blinking , you are going to need a clockspring that goes on the steering column ....they cost about 100 bucks at the dealer ......