I filled it out and got a $25 coupon. Looks to me that it can be used for parts alone unless the "Total service must be $25 or more" means the car must me taken in for service. Bolding is mine.

"One-time use only. Must present at time of purchase to receive offer. Valid at participating Mercedes-Benz dealers. May not be photocopied. Non-transferable between households. Offer applies to parts, labor or accessories for Mercedes-Benz vehicles only. Total service must be $25.00 or more (excludes taxes). Offer not valid for previous labor and parts service. Not eligible for use with any other service offers. Cannot be combined with other offers. Expires 7/20/11. Amount of coupon determined by random selection. If you received this communication in error, please contact Mercedes-Benz USA at 1-800-FOR-MERCEDES (1-800-367-6372). "

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1987 190DT, clackity clack
90 Mustang track car
95 Lightning