ok, so your car is stored outside, it had a nice dash, and it doesn't now. So, the sun beating day in and day out on your car you expected to have no effect?? You ask if you should buy a sunshade, i think that is a fairly obvious answer. I'm not trying to be mean or rude, however quite on the contrary. Many people just expect their cars to stay in the condition they bought them in, however if you do no preventative maintenace(or preventative care), then how do you expect your car to stay in that condition? The condition of the car ALWAYS depends on the care the owner puts into it. My current car see's the outside sun 24/7, however looks twice as good as half the cars its age. All i can say is that isn't because i've expected it to stay that way without any effort. If your car is stored outside year round, it needs to be waxed at least once a month, sunshade put up frequently, and interior treatments done on a frequent basis as well. You can't expect a 20 yr old dash to stay perfect without doing anything to it, mercedes or not.
Ryan

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83 300SD.......sold
96 integra SE....sold
99 a4 quattro....sold
2001 IS300.......sold
2002 330i.........current.
2004 highlander limited....current.