If your Ford rig is setup for keyless entry, The Receiver Will Be Located in the passenger foot well, to the right and above the passenger right foot against the outside wall. It's a plastic box with wires connected on one end. I verified this with my 2007 service manual.
Our 2007 E350 is not setup for keyless entry. We don't even have a wire harness connector there to plug in a keyless entry receiver. I thought maybe I'd get one from a junk yard, but there is no place to connect to.
Of coarse I wished we had keyless entry, but I do wonder how my RV key ring (and my pants pocket) would handle a fob along with so many keys. :)