For whatever it's worth, I hired two people off Craigslist (set it up as a large payout if we closed a deal off their magnets. Made it clear that I had no clue if it was going to work or not) to drive around with our door size magnets one on the driver side and one on the passenger side and a third bumper magnet for about 6 weeks. I hired people that drove extensively through populated areas and parked on the weekends in really busy outside locations. One of them did work with large grocery stores all over the place and was parked throughout the week in busy parking lots and the other one went weekly to a 2-3000 person church in Denver. Both commuted long distances. My plan was to ramp up a large car magnet army throughout Denver if we got a few phone calls during that time. I didn't get one phone call off either car the entire 6 weeks and discontinued the program. However, our magnets were small, we never tried the car wrap and it was only 6 weeks.
I would like to say, though, that I learned that there are a ton of people on Craigslist who are willing to do work for commission only or minimal pay in general. I probably had 25 responses from people who wanted to drive with the magnets. I think, generally speaking, whatever marketing ideas you have, think huge. If most investors are sending 1-2000 mailers a month and the big investors are mailing 1-200,000 mailers every couple months, figure out how you're going to get to the 1-200,000. If you were to find that car magnets are useful in your area, don't just think of putting them on your car, go with a giant car wrap or build the army.