I vote for the pick up truck. Particularly the Colorado. That or one of the small cargo vans like the Ford Transit or Chevy/Dodge equivilants that are becoming popular.
Off topic but how do you detail cars after a 9-5 job? Especially in the fall and winter months when it gets dark at 6:30-7pm? Doesn't leave you much time to work with if you're doing mobile services outdoors.
