If you have an auto transmission, there is a place on the firewall that the clutch goes through, mine is a plastic plate. Easily drilled a hole though it. By the steering column.
Pull up your seats and carpet then do big S loops, you don't want to just coil it up. It won't work very well and I've heard the cable can heat up.
Another place lots of guys place the antenna is behind the cab, mounted to the box. It depends on how tall it is though, it needs to be able to radiate 360⁰ so it would have to be higher than the cab. And a single one looks goofy, if you go that route but two and just don't hook the second one up. They won't be far enough apart and can cancel each other out.
Lots of knowledge on the Expedition Portal forum on this stuff.