lunatic i agree in regards to being able to track the results of your marketing campaign (as you said how else do you know if they are working) this is an absolutely vital part of any marketing campaign...if you can not tell what is generating the interest you are receiving (or what is not generating interest) you can not modify your efforts to generate the most interest in the most economic manner.
however, in this instance and knowing the area that luggnut is in intimately. myself i wouldn't approach this as a campaign type strategy but rather i think it is important for all of the components that make up the image that luggnuts client's (or potential clients) are seeing (his business, truck(s), shirtss, corporate collateral (business cards, brochures, letterhead, etc), website..) that all of these images make up a consistent, professional looking image..that show that he has a presence in the community, that he is established and not a fly by night company. that needs to be the purpose of these projects in my opinion.
and then you focus on campaigns. generating results, tracking the source of those results, modifying efforts and tactics so that you are generating the most interest per dollar invested and most importantly converting those inquiries into sales.
i definitely believe that a wrap can be utilized in that manner and then i would suggest either having a means of identifying that the interest is coming because of a potential client viewing the vehicle on the road (a discount as you said is just one way of doing so) or establishing a separate phone number and website, etc...but again in this instance i don't think that should be the goal of these projects.
the challenge in his area (and surrounding areas) is for him to be viewed as an established, professional company and not a here to day gone tomorrow bottom dollar company. Luggnut is without a doubt one of the premier designers in this area and he needs to become recognized for that ability. There really is no design firms (there are 2...) for potential clients to choose from, if he can become recognized as a design and fulfillment company capable of producing the clean effective style he is becoming known for he will own the higher dollar customers in this market. it is a bizarre market to put it bluntly and big city textbook marketing strategies can put off just as many clients if not more...the market is so small and sensitive that you have to approach it in a manner that you earn clients by doing great work consistently. service those clients greatly so that you retain them and "let" them come to you by being established and present in the community...by 'let' them you can definitely encourage them and do more than open the door everday but direct hard sales tactics are not the best tactic in small communities in my opinion. there is a great amount of psychology that goes into selling in small markets versus the big city. it was a very hard lesson for me to learn...thinking that the same formula that worked for me in a major metropolitan city would work the same in a small town simply on a smaller scale.