[dredmorbius]

Micro FM transmitters. Plug into the 3.5mm jack, power from the cigarette / accessory adapter. Usually a choice of frequencies to broadcast on which can be picked up over your car's FM receiver.

Quite frequently found at highway truck stops, though almost certainly other electronics stores as well.

[semi-extrinsic]

Plus, they're crap. Seriously. I bought one of the more expensive ones, with auto channel scanning, still crap. Made me go back to burning CDs until I put an aftermarket stereo with 3.5 mm jack in. Fortunately that's a near-universal feature these days.

[tunap]

And suffer from interference and low volume; has been my experience.

Wireless is convenient.

Wired is reliable.

YMMV.