Ah yes, sudden inky darkness on a dark twisty road. It does tend to wake you up, eh?

The fix is to clean the internal contacts.  I took the switch block apart to do that.  The reassembly was not fun at all.  A spray cleaner shot into the crevices might work, though you always have to worry about what the solvents will do the the different plastics. 

Yes, the contacts are in the body of the switch.  You have to remove the air bag and steering wheel to get to it properly.  Remember NOT to turn on the ignition when the air bag is disconnected.  That will trip the SRS light in a way that requires a reset tool (and money or some groveling at a nearby shop).

While you are at it, you might want to pull the square cap off of the stalk (the end that you touch) and clean and re-lube contacts in there  with silicone paste.  The original grease can become just conductive enough to convince the wipers to go into intermittent wipe mode.  That "ghost wiper" drove me nuts.

Oddly, none of these things has happened to our E-class which has exactly the same parts.  In trade, the E has pee-ed fuel and ATF on the floor several times while the G has rather leak free.  Again, despite sharing identical parts.  Same number of miles: 192K.

An E-Class turn switch is the same switch, however the cable may be different.  I haven't had both in my hand at the same time to compare.

The easier fix is to install headlight relays right now, while the switch hasn't burned out completely.  (See attachment)  The switch will still operate the relays just fine, but will burn out soon on the current required to run the headlights directly.  That is what I did, and I am still using the original switch.

HeadlightRelayCircuit.pdf (31KB - 26 downloads)

If you use the MB cube relays and the parts listed, then they fit right into the factory relay grid under glove compartment.  The picture won't show much, since the wiring looks totally stock, just with two extra relays and an extra ring connector on the high current cable.

Before you do any real work, check that the problem is really the switch.  Probably the easiest way is to drop the fuse tray, locate the feed for the high and low beam and observe whether they are getting power at the correct time.  

EDIT: I did check out the E-Class turn signal switch.  It looks similar, but cannot be used for the pre-MBUSA G.  It is missing some internal contacts, so you can't just swap over the cable.