Jim, this is the very definition of dirty-rotten-shame! It has a connector, in plain view, and you should be able to just plug it in. Not.
For my 1998 ML-320, every wiring diagram I found was slightly different from my connector. I ended up metering it in order to get it right (this is really a simple thing, so don't let it put you off). I ordered an aftermarket hitch, then used a converter to handle the 5 > 4 wiring issue.
It wasn't very hard to do, but coming up with a plan and collecting the parts was a real pain. Let me know if you want the part numbers I used. I did this a few months ago, so the same parts are probably available.
Edit---I went ahead and looked them up:
Link to wiring suggestions:
http://www.benzworld.org/forums/w163-m-class/36862-electrical-piece-hitch-3.html
I used this power module: (but purchased at a local parts house)
http://www.hopkinstowingsolutions.com/products/basic_wiring/taillight_converters/power_converters/power_taillight_converter_solid_state.html
Draw Tite hitch: (purchased at a local hitch company)
http://www.etrailer.com/hitch-1998_Mercedes-Benz_ML320.htm