I appreciate all the responses. Since I originally posted this topic, I have changed my engine wiring harness and I have not noticed any more sulfur smell in the 3-4 weeks since doing it.
Coincidentally, I had an unrelated problem a few days after swapping out the harness in that the car would not start, even though it ran fine for a couple of days. After bringing it to my indie mechanic here, it turned out to be the fuel pumps (there are 2 of them!). $600 later, the car is now running fine.
I have just purchased a keyless entry kit from Commando Alarms and I'm hoping to do that this weekend. Thanks to this forum for the valuable information about my car, I feel really good about my purchase and about doing the necessary maintenance to keep it running.
TROVERMAN: To answer your question about the Volvo. Our previous cars before the Volvo were a Honda Accord and a Ford Escort. Don't knock the Escort, it was actually one of the best cars we've ever owned in terms of reliability and minimal cost of maintenance. When we got the Volvo, I thought wow, what a solid and quiet car. Now after getting the MB wagon, I feel that the MB is a much more solid car with even no squeaks or rattles anywhere. I enjoy driving both because they each have a different feel, especially after putting in Koni shocks/struts in the Volvo.
The Volvo feels a lot lighter and can launch off the line a little quicker (maybe because the gas pedal is lighter) and it can handles turns very well with the Koni's in there. My wife and I like both cars and the main reason for those choices is safety. I don't think you can beat MB and Volvo for solid build and safety they offer their occupants.