Karen Ann,
I could be completely wrong, but IMHO it looks like you missed the most important step in launching a QR Code campaign. It appears that you don't have a mobile strategy. Sending consumers to inventory on a desktop site from a QR Code defeats the purpose. Before spending time/money/effort on QR Codes, I would implement a strong Mobile site.
If I'm a consumer scanning a code on a vehicle, I expect a lot of info on that specific vehicle, photos, carfax, comments, contact info, maybe videos, etc.
I do have one question, how are you getting the QR Codes on magnets?