Problem with volume control and flashing display on Ford 6006 Cd/Radio?

I recently replaced my daughters cassette player in her V reg Ford Fiesta with a Ford 6006 cd/radio that I had previously removed from a Ford Galaxy. When it was fitted to the Ford Galaxy everything worked fine but since fitting it in the Fiesta the volume control is erratic, as you turn the knob clockwise the volume goes up and down and the display constantly flashes. The unit plugged straight into the connectors on the Fiesta so is there something corrupting it in the wiring. If anyone can help with this, it would be greatly appreciated