The wipers completely stopped working on my '98 ML320. I can hear the motor run, but the wipers do not sweep. I've read postings about this issue on this Forum but I want to be sure I'm not missing anything before I either take it to the dealer or DIY... If the motor is bad, why do I hear it run when I move the wipers in the "on" position? Thanks for any help...

 

With the engine off, grab one of the wiper arms and try to move it upwards. If it does, either the wiper transmission has become detached or it is broken.

 

I've had a lot of troubles with the front wipers. If you go to the dealer I'm afraid you will be robbed shirtless. There is two things that go bad there: the wiper transmission and the motor. Now the motor is not an either-or thing, it can have the many various grades of bad. It can become slow, it can stop working in one selector setting but not in the other, it can work on and off making you think it's a wiring or a relay issue, but it's just the motor.
The transmission may need cleaning, you might want to put it in kerosine/diesel fuel or other cleaning solution overnight, there may be oxidation, rust, or debris in it that make it freeze. I had mine changed for a used one that was brought back to life in this way, the motor I got new aftermarket made by Meyle for like $120 or so. It's not a simple DIY, but it can be done at home, there are probably instructions on forums or on YouTube or something. Takes under 30 minutes if you know what you are doing.