Are there any studies that show the effectiveness (or the opposite) of ads placed on cars (wraps, magnets, stickers)?
I think it would depend on the business. I haven't seen any studies for car wraps or graphics but I have clients who have this done and it helps in many tangible and in-tangible ways.
Main Negatives I see is if you do it for the wrong type of business. But, I think many businesses it will work well for. I tend to find tasteful wraps much better than just stickers - which seem cheap / poor. No one wants to do business with someone who isn't successful. So if you do it, do it right and get it done professionally.