1999 Windstar RSS (reverse sensor system) help needed

Hi everyone, I just bought a '99 Windstar SE with the 3.8L , it has 69K miles. Everything on the vehicle is working great, except for the rss system. When in any gear other than reverse I can switch it off and on (although it doesn't work), but when I shift into reverse, it switches to off and cannot be activated. Does anyone know what this could be? Any help would be appreciated. Thanks, Jeff