Hey Bish, I love how that turned out, all grimy and muted colors!
And normally I don't dig the look of vehicles being placed parallel to the base sides, but something about your did composition seems to work OK in that regard. The figures are looking really, really good too!
When you go to do wet mud, use Celluclay, and smooth it out with your wetted finger and then put in your vehicle tracks, footprints, etc. put in some decent ruts and divots in the road--these are effective for catching "water" and helping to carry off your impression of wet ground.
Now, this is important: wet muddy ground is never universally wet, nor glossy, and even if you think it SHOULD be, artistically it doesn't come off well to do that. Paint your ground a rich chocolate brown or the. Like, and then dry brush some lighter shades on the high points, just like you would on a model.
NOW use Testor's regular gloss lacquer over it. Get a bottle of it and airbrush it. Give your road or earth a LIGHT coat at first to seal it and prime it --it should be no more than a semi-gloss. Let that dry. Then brush thicker applications of the gloss into the grooves, track and foot depressions, and into the lowest points of the road. If you have deep channels in the road, or a ditch pool for example, use 5-minute epoxy to make "water"-- it will dry clear. You can also then tint any part of that or even the gloss channels with a light wash if you want to show color Lin the water, and then put more gloss over that to get depth.
lastly, dry brush again if you think the effort needs it. You should have a roadway with wetness and wet channels in it with higher, "drier" parts giving visual interest and contrast. If you over-gloss the road, it will look unnaturally shiny, like you simply lacquered the road.
Good luck! I hope it comes out. How you want it! :)