I'm not sure that it's the different "plant" idea ... at least on Gen 1 cars . I think it was more of a "high-end" to "lower-end" cars . The higher-end cars with more bells & whistles will have the relay socket (most likely due to a "power group" option) , where as a "lower-end" car will not (maybe a Zetec , manual , roll-up windows , etc.) . I've looked at many people's 'Tours & that's the only type that I've seen (personally) that doesn't have the plug .
Every SVT I've ever looked at HAS had the plug . On a similar note , my mom's '98 Mystique (zetec , but fully loaded) DOES have the plug .
~~~~~~~~~~~~~Allen