You're basically 'shotting' a few cars for a bit of cash here and there. If it's a few hundred quid a month you're after just carry on doing what you're doing. Truth is you'll probably not go back to it once you graduate, I wouldn't recommend you do anyway, there's far more money in drugs than cars... however, alot of money in cars is from drugs and alot of people who buy them are on drugs. Maybe there's more connections than I'd first thought
Anyway, there's no silver bullet, just keep plugging away and ask on here if you get stuck with any nasties.