Dirty Harry,
I have a client/friend with a similar wash that is in the launch phase.
This owner plans to explore this option, adding services like interior vacuum, express wax, etc., at sometime in the future after he gets his feet wet and arms around the business of washing cars.
I also have a client/friend with an express exterior tunnel with free use vacuums. He has been open for several years. His decision to offer detailing services was at the continuous insistence of his customers. He has been slowly introducing these services incrementally.
He started out with a hand-prep service at the tunnel entrance. About 25% of his customers are buying this at $4.00 a pop. Recently, he introduced a wheel deal service that includes hand-prep and toweling drying, tire shine and brake dust protection performed in the vacuum area. Out of the gate, about 10% of his customers are buying this at $8.00 a pop. Later this year, he plans to offer an interior vacuum service and probably a hand-wax service.
Whether this is would be in the best interest of your client would depend his and your previous experience, current performance of the wash and the local market; segmentation, competitive environment, climate and other site location factors.
I would not want to suggest how to proceed without knowing more about the situation.