What's the best way to wash/rinse the undercarriage of a low sitting vehicle like a mustang? With my explorer I usually just go to a spray and wash and can easily get the sprayer under the vehicle.

Not so much with the ground clearance of the '13!

Are the undercarriage wash machines you find as an auto car wash useful?

 

What are you trying to accomplish my washing the undercarriage? If you are trying to get road salts and such off of them so your car doesn't eventually fall out from under you then there are a buttload of products designed to neutralize the salt and chemicals that are used to clear the roads. Most you mix with water and spray on using a pesticide sprayer (new or cleaned of course). Other than that putting your car on a couple of ramps and spraying the underside with a pressure washer is the preferred method. By me anyway and is a once a season deal.