I recently picked up a nifty FM transmitter, which either will accept
input from an external device via cable, or plays MP3s directly from my
thumb drive

it plugs in securely in the cigarette lighter (in the camper i can even
use a rear cig lighter)

I bought mine @ Mall-wart for $29 but i've seen them for more @ popboys
for $39 on sale..

heres a quick little article on it.
http://gizmosforgeeks.com/index.php/articles/849
"No mater how you store your MP3 files - MP3 player, memory drive, CD
or any audio device, the Virtual Reality Sound Labs new VRFM7 plays them
wirelessly through your car stereo using your USB flash memory drive.
Simply plug the VRFM7 into your cigarette lighter and then plug your USB
stick into the VRFM7. Compatible with iPod's, MP3's, CD's, portable DVD
players and laptops. Includes 7 LED preset FM indicators and standard
USB port. Channel select, play/pause and track selection controls. A
supplied audio cable connects to all popular audio devices."

for better reviews on the product do a search @ www.google.com for
VRFM7