The diameter of the cigarette lighter in my 1999 SLK 230 is too large to use any equipment (GPS, Satellite Radio, etc.) that must be plugged into the cigarette lighter outlet. Is the some kind of an adapter, extender or other gadget that would allow me to use my electronic devices in the car?

Yeah, MB cigar lighters are larger diameter than most others. I've found you can make the adapters which have the spring loaded bands on the side work but you have to wiggle them around to get a good center contact. If your device has some sort of LED to tell when power is on that's a big help in knowing it's working.

Not aware of any work around but that doesn't mean there isn't one.