I just installed a new DVD in-dash stereo with rear view camera and discovered the factory speakers in the doors of the 2003 E450 were nothing to brag about. Less than $40 and 20 minutes for an install later (that includes both doors!), the new Pioneer speakers sound great!
I went from factory 20 watt to Pioneer 250 watt (that were a perfect fit to boot!), and what a difference it makes. The install was extremely easy with the assistance of Ford forum post I found. Let me know if you need a link to the directions. I HIGHLY recommend getting the $5-6 adapter that connects directly into the Ford wiring harness and onto the speakers...it couldn't make it any easier and no worry about getting the polarity wrong!