I ran into the same thing on my wife's ML320. You have two lower cost options to repair it that are a lot cheaper than the dealer:

1) Find a shop in your local area that does speedometer repair for autos. Most of these should have the ability to repair the trip computer. The local shop here in Jax FL is A1 Speedometer, who will repair one for (6 month old estimate) $100-$200 depending on how much needs repairing.

2) Check ebay. I found a NEW in the package trip computer last week for $95 plus shipping. Just put it in Wednesday, and it works like a charm.

Don