One guess who pays for the car, insurance, tax.... silly dad!!
Give the two of them a boot up the backside and take away the keys to the Polo. If that doesnt shock them enough to stop fighting over it, nothing will. Make it clear that as long as they can't agree on who uses it when neither of them will have it.
And if they need a car that badly let them go off and buy one of their own.
When they were younger and fighting over who played the Nintendo when, did you run out and buy a second one for them?!?!
This is a classic case of daddy-the-banklink and you're the bigger fool for going with it.