Date registered: Feb 2007
Vehicle: 78 MB 450 SEL
Mentioned: 2 Post(s)
Quoted: 592 Post(s)
Troubleshooting electrical problems such as this are notoriously difficult for someone who isn't experienced with these types of vehicle systems. If the home mechanic can't locate the problem by looking for loose connections, blown fuses or other obvious causes then it's virtually impossible to solve the problem through an online forum.
Fortunately, there are many professional mechanics who are experienced at electrical troubleshooting and, for them, fixing a problem like this should be routine. Speaking from personal knowlege, they've helped me solve problems similar to yours pretty quickly and without too much expense.
Look for a shop that specializes in ALTERNATOR service and see if they'll do the job. They usually have the tools and the experience to methodically hunt down and isolate electrical shorts, and this typically doesn't cost much in time or parts ($). Ask around or do an online search or even look in the phone book for a shop that can help you.