Actually your General Electronics Module (GEM) by the fuse box is also your RKE module too.
It is all setup to work with the hard wired keypad in the door. It just needs to be added.
I added a keypad to my Taurus in May. Was not easy. but possible. MOST ford models after 2000 that have the keyless fobs, have the capability of the keypad. Just no keypad. Taurus /Sable is easy becasue no cutting of the door sheet metal.
Adding Key Pad To Gen Iv - Taurus Car Club of America
The Wireless ones that old wizard speaks of were peal-n-stick.
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2003 Ford Taurus SES.
3.0L Vulcan Engine.
1998 Ford Explorer Sport - Sold - Bad Tranny
4.0L Cologne Engine.
1991 Geo Prizm - Rolled it

1.6L Something or another
All of the information that I provide are from either personal experience, or documents I have read. Please perform them at your own risk.