I hate electrical problems (w210)
 

I have the running lights out on the driver side front and rear. I have checked the sockets for continuity, the wires for resistance and the fuse. All seem to be ok.

What else could this be??

Not that much information to work with but, maybe you have a bad ground. I havent had to work on mine yet.

Grounds,Grounds,grounds,grounds
 

(Have to supply 'own tune to hum it to.)

Trade the W210 in on a nice W124?

Grounds, grounds, grounds, grounds, wonderful grounds! (Like Spam, spam, spam...)

Either there is current to the lights or there is not. An Ohmmeter should reveal this. Either you have a good ground or you do not. If you have current, the ohmmeter will indicate the ground. If you do not have current, you can reroute an external wire to check the ground.

Odds are, it's one or the other, unless the bulb or the socket are bad.

Everyone hates electrical problems. It is my view that the basic problem is due to the fact that electricity is invisible.

In my 124's, I can select which set of running lights I want, left or right by turning the headlight switch counterclockwise from center. Could your switch be in this position accidentally?
Anziani
'93 CE
'95 E420

Mine has needed new front bulb replacement several times. But not the rear.