Some car radios have an AUX IN plug that looks like a headphone jack. Perfect for connecting headphone-out to AUX IN.
If yours doesn’t, you can spend about $30 on an FM transmitter that will plug into your headphone jack and send a signal to be picked up by your car radio. The transmitter will either broadcast on some pre-chosen frequencies or let you choose the frequency to find an empty spot on your FM dial.
My experience with an FM transmitter (Belkin) is that it works OK for a while and starts to fritz out when the signal drifts. But if you’re desperate, that’s your option. Or, for about $120-$150, you could get a car stereo with an AUX-IN.