Another alternative is a small 3.5 mm to Bluetooth device. It allows for any 3.5mm output to use Bluetooth and best of all: already paired and battery is on the module not the headphones themselves.
Depends on how "old." Just as pretty much every car finally had an auxiliary input (even the cheapest rental car), Apple deleted the headphone jack from its best-selling music player.
And now we've regressed to no aux inputs, no audio outputs, and nothing but shitty Bluetooth. But the '90s Pioneer head unit in my car has an input, as does my stock 2009 Ranger.
Nice option for (old) cars.