I just went through this in fact.

First thing you do is don't capitolize every word in every sentence..reflects poorly on you

Ok now seriously.

Get three to five very good pictures of your car (borrow a quality camera and a good shooter) and include them with the request.

Get a resume up for your car. This should include (depends on if for show or race btw):
Timeslips...not the actual timeslip but Final time, 60' time, and final speed. These are important because if it is for race they want to see consistency or impressive numbers. Make sure they are accurate and include date and location of the times.

For shows include any shows you have been in, what class you showed in, and how you placed in shows. Local shows and national sponsored shows alike it tells them you are a competitor. Include location and date so they can verify these things.

Modifications to the car. No detail is too small, although including things like the fact that you color coded your plug wires is a bit excessive...use good taste in listing them but do not skimp either...they like nice lists.

A little about you. Nothing major just include a bit of your personality in the resume....how you present yourself is a big part of whether they are going to consider you.

If you are presenting this in an online format then include links to any and all coverage of your car not done by you (do not show them your HA look at my car posts). Online magazines, ride of the month, online listings of times and peak performers lists etc.

Include with your parts list a list of the shops you have done business with...shop loyalty is a plus mark as it shows consistency in how you do business.

Last but never least is to include a list of what you are going to do for them. These people are not going to hand you a $1500 set of wheels or tires just to let you drive around town on. List all shows you plan on showing at including dates and locations, all race events you will be participating in and be specific again, any runs you attend on an annual basis where their product can or possibly will be represented in some form or another...include local poker runs and parking lot shows as it shows community involvement (big plus).

Trying to think if I left anything out....OH I know...contact information should include the following:
Name (duh)
Location...state and city is enough
Phone number (duh)
Email address....do not give out a stupid email address like pookybutt420@smokebud.com ...keep it proffesional and get a real email address.

I think thats it anyone feel free to add to that if they wish.