Chris, I'd check over at Crutchfield to see what specific configs are available for your vehicles.
As John notes, these FM transmitters are pretty common and work OK. Per another discussion on this same topic a few weeks ago, I purchased an $18 unit on eBay that I've been using in our W210 cars. I guess it works as well as can be expected. Some brands are junk, this one brand Accumulator mentioned seems to be slightly better. These devices don't work well when you're in a major metro area.
Also, BIPES for the Benz:
Bipes Automotive iPod / Portable MP3 Adapter 