I can vouch for the new car wash located on your right on the way out of town, a couple blocks before highway 200. I forget the name of the place, but just look for the nice new white building and the drive-up ramps for washing your undercarriage.
I got a very good car wash with a thorough inside cleaning and vacuuming for only 70 pesos. At least as good a job as the bigger Melaque places, for a fraction of the cost.
The owner Juan is very nice and knows a fair bit of English. He doesn't have a phone number or business cards yet. His normal opening time is 10:30.
He also offers:
> Undercarriage pressure-washing
> "Vasolinada" (a very light grease thoroughly sprayed on your undercarriage to fight corrosion.)