News wrote:

eBay item 291474623251 which plugs into the cigarette lighter
and plays from SD card. I think the idea is the output is picked up by
the car radio, and plays through the usual car speakers.

Have I understood correctly?

Yes, the basic "iTrip" functionality, it's an FM transmitter.

Any other suggestions?

You can also get MP3 players that are built into a cassette shell that
play into the car's stereo via the tape heads

e.g. http://www.ebay.co.uk/itm/281627386734