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Yeah there's lots of options, the cheapest and simplest is one of these:
https://smile.amazon.co.uk/dp/B00KON1JIA/ref=cm_sw_r_cp_apa_fab_DOAGFb53Q085Q
It converts Bluetooth sound to analog and sends it to the car stereo via the 3.5mm port. You'll have to control everything either via the phone or the little receiver, but it'll be reliable and decent enough quality.
NurseHolliday-
I've had mixed results with those things. Having a battery is a potential liability, and won't stop when you switch the ignition off etc, but the one I had without a battery used to glitch and lose the connection, probably because lighter sockets are an awful way to power electronics.
If you have an iPhone there are cheapo lightning to audio adapters that also allow charging.
New (very old) car didn't have a line in, so I bought a £10 (!) BT mechless head unit and it's the best experience I've had in the post-headphone socket era.
grams
This seems like the best place to ask. Is there an easy/cheap/decent way to connect my phone via Bluetooth to my car's stereo? It has a 3.5mm port and a USB port.