I wanted music from my smartphone in my car and a hands-free set.
I had an old (warrantyless) car radio with aux in (but no matching connector). An old phone headset nearly falling apart but working, some thick copper wire, a soldering iron and some sheet metal scissors.
The soldering iron solved the connector problem. Since I did not want to take the whole thing apart for just 3 solder joints, the scissors were useful.
The headset is now a fwd/pause/rev button with hads-free microphone. It got a gooseneck to be close to my mouth for better sound quality.
It looks strange but works nicely. Android in the car for $0.