There's a non-greasy Armor-All formula that's great for plastic dashes. Seems to last a lot longer than any other like-product I've tried.

If you've got completely faded plastic - like those 80's VW bumpers, I used to use black shoepolish. Messy job though.

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'64 230SL

meguairs has a vinyl shine ina red bottle. it works great on the benz dash. I have used it a few times, and it looks great smells good, and works extremely well.

I am lucky, meguiars has a truck that stops by my work every couple of weeks.

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'92 300CE - Sold
2004 C240 - 744 - C7 Wheels - Android Radio
2002 C320 - 816 - Sport Wagon

Sunlight Infrared (not uv) sinks into any dark object and dumps energy into the material. A dash carpet will take most of the heat and prevent the dash from reaching such high temps. The heat is not conducted because carpet is a good insulator but I realize that in some places the heat might build up in the interior space if the windows are shut tightly and that may contribute to a warped or cracked dash.
UV might help decay plastic molecules though, they say it is the principal cause of skin cancer.
Mercedes are not the only cars with cracked dashes. Occasionally I use a Nissan Pickup that has a Horrible dash, the stuff inside is poking out of some deep fissures where its cracked, Ugly!

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!