Why did he have to buy a new one then? We've already got a car stereo. (I'm sure you did too.) There must be something the new stereo does that the old one didn't? A different input socket?
Mmm, maybe I should phone one of those shops you describe - if it's just a q of fitting a lead it could turn out much more cheaply than I'd thought and I can spend the rest of my birthday money on emulsifiedfats books.