i find that taking a tow truck tends to help the situation, because what you then do is chop the handbrake cable of the car you are going to take and hook it up before knocking on the door

then, when no one replies or he's "not in" you simply leave an address where the remanants of the car can be collected from once you have sold enough of it to recoup your costs

and if they guy comes out and says "here you go, here's the money" etc etc "now fix my handbrake cable" you simply explain that the cable cutting was free of charge and bid him farewell

my cousin did this to a guy many moons ago and left after giving him a cheery wave

he also opened some guys V12 jag up, popped the bonnet, attached a set of leads to the relevant electrics to wind the windows down, put them down and then removed the battery and all the fuses after the guy failed to pay him for fixing the car
he was sheltering under an umbrella at the time of doing all this but the car was a bit wet and pongy as the guy didn't have anything to stop the water geting in for the next few hours