take the radio out and see what is causing the problem??
is it the plastic front plate binding into the push button??
if so then adjust the front plate... if not then remove the front plate and go to radio shack and buy a $3.95 can of
electrical contact cleaner and spray the volume control and see if it works now, dont reinstall the radio, just attach the electrical up and move the vol. control back and forth to see if it works.. if you need a new(another) radio you can go on E-bay and get a stock ford radio for about $12.00
and shipping.. hope this helps.