Looking at the water filters, the shower water filter won't work at all with a garden hose (no pressure washer) since it doesn't flow very well.
I was looking at these RV filters and they look like they'll flow better and easier to install since you don't need any adapters. Some Amazon reviews say it works well for car washing too.
http://www.amazon.com/Camco-40043-Ta...pr_product_top