Most of these places offer you a discount through several vendors. They do give you a discount but not better than you could get if you shopped around. Plus they got your money to start with.

I'm with Alpha I had to just about finish my car before I got sponsored and then most of them wanted me to jump through hoops and put crappy stickers everywhere.

You are best to take some really good pictures of your car and make a portfolio and send it to just the people you want. See what they want you to do for sponsorship and see if you can live with it.

Memphis Car Audio is great. They gave me over 3k worth of their stuff and only ask, "If there is a car show in your area can we have your car in our booth, if it is convienant for you." Like Adam Sandler said, HELL YEA!