"how to safely get at the positive pole on the alternator with the engine running? It seems rather tight in there. "
You should install the wire on the alternator with the engine shut off, making VERY sure that it will stay in place and not short out when the engine is started. Don't even think about trying to run any wires or any diagnostic tools into a running engine if there is ANY possibility that they or you will contact anything that will hurt you or the Mercedes. Keep in mind that if you get a shock, you will naturally try to get out of the area and perhaps will contact something that is turning and really hurt yourself.
Perhaps a primer on general safety procedures might be a worthwhile thread to start so we can perhaps save someone more than a skinned knuckle or two. Frankly, such a list would be a good thing for us all to read over occasionally as we frequently take chances that we really shouldn't.
'85 300 SD, 165K-Sold
'88 300 TE, 165K-Sold
'64 Porsche 356C Cabriolet
'86 560SL 124K Miles
'94 320E Wagon, 74K Miles