There is no quick and easy answer I`m afraid. Firstly check the obvious...Turn the radio on , then find where the wiring passes into the door and "wiggle" it to see if that has any effect. If it doesn`t hen it will mean removing the door panel and checking the speaker+wiring back to the radio using a spare speaker and a multi-meter ( If you have the security code and have access to another Ford Radio, then swopping it will prove whether the fault is the actual radio or the wiring from the radio socket to the speaker....As I said , there`s no quick+easy answer (especially if it`s not a damaged wire between door + car )
RegWorts
May 2008