im not too sure on the interior of the 97 but this is either a, corrsive build up on your speaker wires behind the deck or possibly they deck itself is just crapping out on you, pull out the deck some fords have 4 holes around the stereo area thats for the "keys" they're usually in your cars owners manual, check the front speaker cables see if theres any corrosive build up or broken possibly ( there all labeld nicely back there you wont have any problems) if all seems fine behind the eck i would say start going to look for a new deck