How can I clean a really dirty truck rug?

If you can remove it, place it on the ground and wet with a garden hose. Pour an oxygenated cleaner like OxyClean or Awesome Orange (my favorite) on it and scrub with an old towel or t-shirt. Hose again until the water runs clean. Hang to dry.

I would start by spraying it first with some really hot water and scrubbing at it with a brush attachment, or even a scrub brush. Then use a shop-vac to remove as much surface dirt with the water as possible. I bet it won't be a fast project and you'll have to do it several times. You could use a light soapy water at the beginning to break up the dirt but don't go too heavy because rinsing it out would be a bear. An auto detail place might be able to offer that service, you have to determine how much is your time worth trying to clean that manually.

Get another rug if you get small mats they usually fit it in same spot

Take it to the car wash . Hang on rug hooks and wash with suds and rinse. No waxxxxxx!! Teehee