It's in the trunk latch mechanism. If you have a plastic cover, you'll need to remove it.
If you have the haynes manuals, the switch looks identical to the switches shown in the doors for sensing the door is locked, IIRC.
Check with the Ford Parts counterman, he should be able to show you where the switch is when you buy the new one.
I replaced mine and it was less than $10, IIRC. And that's how I learned where it was too, from the parts book.
TB