I was once a serial test driver. My method was pretty simple. Phone and ask about buying one. Ask about trading in an expensive car. I have a "Mercedes ____ and was thinking about the 911 as a replacement...". Talk about prices and options. Don't mention test drives. Say you have your eye on a certain model. It'll be a company car, etc etc. By the end of the conversation, as long as you've given the impression that you'll probably be buying any day now they'll invite you to come and drive one. Look smart when you turn up and whatever you arrive in hide it round the corner.

After the drive is when you face the challenge of fending off the sales people who will ring you repeatedly and hassle you.

Charlie