I have been professionally repairing and restoring european auto interior trim for over 31 years. the best molded dash covers are made by accuform not coverlay. I have installed hundreds.
avoid all armor_ll type dressings they destroy dash material by accelererating the out migration of oils in the vinyl skin.
severe front to back cracks are very difficult to repair in a way that will last because of the thermal expansion and contraction of foam filled dashboards. the crack becomes an expansion joint under strong surface tension as the dash warms up.
a really good repair approach is to buy a medium bottle of thin, fast setting CA or super glue from a hobby store. wipe the cracked area with some alchohol, get a flat metal tool that is a couple of inches wide, soak into the crack with the super glue and press and rub the area with the tool as it sets, keep the tool and your fingers moving or they will lock to the surface, as the CA soaks into the foam and kicks off QUICKLY it may generate some noxious smoke that could sting your eyes, rubbing the area with the tool pushes down the edges of the crack that has usually raised up, once the glue kicks off in about FIVE seconds or less the edges should stay down. The CA glue should soak deeply into the foam core and stabilize the area sourrounding the crack . The whole approach is twofold structural stabilization and then cosmetic. The cosmetic part is to level and fill the crack then texture then color and blend. 180 grit wet and dry sand paper is used to level the high area the crack has caused. you can use the sanding dust to fill the crack and soak with ca or baking soda and ca. so fill soak sand and repeat until the crack is filled and level. try to keep your sanding etc localized to the crack area. texturing is an art that takes a long time to learn. one off the shelf product that works is found at body shop supply stores, and is heavy bodied material in a spray can. I think it is designed for exterior rocker panel areas to repair the textured area there before painting, practice on another flat surface to approximate the grain in your dash, usually if you spay lightly at distance you will get a good result. one trick is to tear a 1 inch round hole in a paper towel or piece of cardboard hold it one inch above the repair surface and spray thru the hole onto your dash to control your spray area and pattern. let dry and lightly sand wth 400 grit wet and dry to adjust your texture. the best color coat out there is SEM brand vinyl sprays in a rattle can. found at body shop supply stores in lots of colors.on mercedes you will most often use satin black. prep the whole area with prep-sol/ wax and grease remover, after your done you can re dress the dash with some good dressing from meguiers or the like. practice with the ca glue and baking soda on some thing else to learn the sand/soak/sand process. Big freebie.. the ca glue/baking soda process can have lots of applications on hard plastic repairs and more.