The only people I know who don't use headphones are both teenagers and reviled by the people around them. If what you claim is the case, public transport would be a wasteland of pop music and clash of clans sound effects. I highly suspect your friends aren't representative in the least of the general population.
I use the phone's internal speakers to play music in the car. I would use the audio jack for that, except that cars no longer seem to come with any way of accepting an audio cable. My car from 2002 had a tape player, and I was happy using an audio cable with a casette tape adapter on the other end. A car from 2015 just can't do it. :(
I also use the phone's internal speakers for playing the notification sound. (And, technically, ringing.) I sure hope that's not going away.
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.
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.
Doesn't your car have Bluetooth? Or rather: how old does a car have to be before it doesn't come with Bluetooth as standard? Every rental I've had in the last five years has had Bluetooth to my recollection, and I don't hire expensive cars.
As pointed out elsewhere in the thread, bluetooth is not a good solution when you want to use one speaker (the car) with several devices. The beauty of a cable is that it's unambiguously connected to whatever it's plugged in to, and not to anything else.
I recently had the joy of having to delete a paired device in a car before I could add a new one. For some reason you could only have three devices paired.
Does your car have a USB port? If it does, it might speak iAP to play audio through it. If it doesn't, you can always replace the radio and get some more inputs.
Bluetooth's more reliable than USB in a car anyway.
