Does anyone know how to repair the automatic antenna for a '78 vette? Just stopped working and it stays in the up position when you shut the radio off.

1) With the engine off and the key in the acc. pos. do you hear the power antenna motor running?

2) If it is running then the electrical side if the circuit Probibly OK. Try looking at this diagram about changing the rod assy.
http://www.ecklerscorvette.com/assets/pdf/corvette/29285.pdf

3) If the motor does not run then it's time to look for dc at the motor, from the radio. Don't forget, with a plastic body, it could be something as simple as a bad ground wire.

It's been 20yrs since I chased one of these so I hope this helps...
Good luck