General wear and tear on a willy's jeep.....?

The easiest way is with pastel chalk dust. Get some pastel chalks at any decent art supply store (don't get the oil-based kind, just plain old chalk). Rub it on some fine grit sandpaper, pick a little up with a brush, and brush it on the model. If you don't like the look, it will rub off. Of course that is one of the drawbacks, it does rub off. I've sprayed a flat coat over it and not lost the appearance, but a gloss coat did cause the appearance to get lost. You have to spray at very low pressure to or the dust will just blow off.

Lighten the base color with white, tan or even yellow (the jeep is painted in olive drab isn't it?)
No mud? Unless it is used only on asphalt roads the mud will be present.
For a frontline jeep there was hardly a tiime for a proper carwash.

If you use Conte Crayons (don't know if you can get these in the Colonies), they can be used in a similar way to pastel chalks, but don't blow off when rubbed/ brushed into place on a matt surface. Also, if you don't like what you've done, you can rub them off with a putty rubber, and start again.

Chris.