When I bought my wife's ML320 in October 2001 (its a 99) the left rear indicator wouldn't work. I suspected the bulb but a replacement still didn't work.

While troubleshooting the problem, I found one of the connector pins for the rear light assembly was bent and not making a connection.

I don't think that the rear LH indicator light had ever worked and I don't think that anyone other than following drivers had ever noticed!

Watch those connector pins, they are easy to bend.