I do washes for 3 different businesses in their parking lots. I do it on the side of the building with no traffic except employee parking. In 2 cases one employee had my services done and pasted the word around they building and now the owners let me setup shop in their lot. The other got my name from a client i did and the owner asked if I could come do washes in their lot for the employees. Only problem I ever run into is trying to fit in as many cars as you can and still let them go to lunch.
Talk to your customers and ask them, if they are pleased with the work please spread the word. If you get any business off it, throw in a free wax job next time that client gets a wash.
Get long extension cords and water hose, and a 12x12 canopy!