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Check out RepairPal's article on the check engine light: http://repairpal.com/check-engine-light If the light is blinking, it can be pretty serious. The big risk is that whatever is wrong will ruin your catalytic converter, if you keep driving the car, and this can be very expensive to replace. Go to a shop and be sure you get a confident diagnosis. If they throw parts at this, it may recur and cost you more money.
I think it's highly unlikely that the dealer would do this for you. You may be able to find some illegal aftermarket fix. I recently read that the auto makers, including Mercedes, are looking at solutions that would allow for the passenger to watch while the picture is hidden from the driver--so they are actively working to prevent you from doing this.
The manufacturers of accessories like this are really good about identifying which vehicles they will work with. My advice is to go to eBay Motors and filter your search results by your make model and year. eBay will only show you products that fit your car--that will give you some guidance.
Yes, you have to go to the dealership for new keys, as they have to matched correctly to your ignition switch/cylinder.
I recommend going back to the shop that just did your service. It is their responsibility to complete the service or repair and correct the codes in the car's computer.
You can sometimes find this sort of information and diagrams to help you at www.autozone.com. Try the "Vehicle Repair Guides" link on their homepage.
That will most likely have to be done by the shop. Did you have the service performed? Benzes are computer heavy, and I wouldn't be surprised if resetting the light needed to be done with a scan tool.
I've heard that the GM security keys have a tendency for the little security chip, which makes physical contact with the ignition switch, to wear down over time. This can cause this kind of intermittent problem. The first thing to try would be a newer key if you have the extra set still, or to visually inspect it. If that's the problem, you'll need to get a new key from the dealer.
I've heard Interstate over and over again.
Does the key chain remote have a battery that can be changed? That is the first thing to check.