BikeRider,
One thing you have to check beforehand is to see whether your 03 Expy has a 16 or 24 pin wiring harness. Ford made the change in mid stream and failed to tell anybody. I found that out when I ordered an MP3 HU for the truck from Crutchfield recently. They sent a 16 pin harness with the radio which turned out to be the wrong part. I didn't find this out until I had already wired the harness to the replacement radio, THEN pulled the factory radio out. The dealer will not know the particulars as to which harness you have. You must pull the radio out yourself to find out. This takes about 5-10 minutes total and is very easy.
The CD-6 receivers look the same, but do have different harness connections according to the truck/car they're originally installed into. I made this mistake some time ago when I bought one on Ebay that came from a Mustang but was said to fit in my Explorer with no problem. It did fit, but sounded like it was inside a muffled tunnel. Turns out it was meant for a truck or car with mach sound and a "separate" amplifier.
If it turns out you have the 24 pin harness in your truck, let me know and I'll post the Ford part number for the CD-6 which goes in the XLT.
As for an AMP under the 3rd seat, smo0othride is right. There's no space under the seats at all when folded down.
TL