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Just seen eBay item 291474623251 which plugs into the cigarette lighter
and plays from SD card.

Yes

I think it also plays from a phone.

Don't know about that, but you can plug in a USB stick with .mp3 files
on and it will play them

I'm not
sure how it works,

it transmits on an FM radio frequency which you tune your car radio ti.

but I think the idea is the output is picked up by
the car radio, and plays through the usual car speakers.

yes

Have I understood correctly?

yes

I have ebay 301571807011 which works fine, but the problem is that it
doesn't make good contact with the ciggy lighter socket in the car.
This is a problem I've had with other ciggy lighter socket gadgets,
including phone chargers. It's probably the fault of the lighter socket
in the car rather than an inherent problem with the stuff I'm plugging
in.

Gadgets like this also tend to work their way out of the socket when my
car goes over bumps. Again, probably very much dependent on the
particular model of car and the ciggy socket used. I'm thinking of
adding a couple turns of electrical insulation tape to the body of the
mp3 player to help it stay seated in the socket.

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