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Door sensors cannot not be inspected visually. Be sure the door is closing properly and fully. Test it by closing the door with vehicle in park and running. Push (wish some good force from the inside) on the doors and see if the sensor trips and you will find the one that is failing. Most likely on the passenger side of vehicle.
It could just be the contacts on the switch have gone bad and the switch needs to be replaced.
Agree with #1. Possible alternator issue. You can check for continuity in the line with a multimeter. Touch the black end to black on the battery. Touch red to the lead connection on the alternator. You should get a read equal to the battery itself when not running. If it reads 0, you have a break in the line and the power is not getting from the alternator to the battery. Start vehicle and touch black to black on battery and red to red cable on red side of battery. While running, this should be about 14volts. If not the the alternator is the problem.
Need to have tie rod ends and ball joints inspected as well as front struts. This is best for a steering and suspension shop to diagnose.
Agree with#1. Probable broken cv axle joint. Steering wheel shakes and you will usually a grinding and "marble rattling noise".
Check sparks plugs. If these are okay, I would suspect a vacuum leak to one of the hoses on the intake. Usual suspect is the pcv hose or the rubber elbow on it. You can spray carb. cleaner on the hoses (while vehicle is running) and listen for the engine idle to change. If it does, you have found your vacuum leak.
Here's the procedure for you: http://www.ehow.com/how_7476619_replace-mercedes-oil-pan-gasket.html
Is the check engine light on? If so, please post codes. Also, check rubber air intake hoses for any cracks or holes (PCV elbow hose at intake manifold is a usual suspect) Has any maintenance been performed like spark plugs changed or ignition coil replaced or o2 sensors?
Agree with #1. Needs inspected. Possible filler neck rusted out or the rubber connecting fuel hose.
Agree with #1. Repaired a Jeep cherokee sport with same issue. Slightly smaller belt took care of the issue. I think the spring tension in the tensioner was wearing out causing this issue.