I'm with fasthair here.

Check your alternator. Take care unplugging it with the battery connected. Is is possible to fry some components if you "jiggle" the connector on he way in or the way out. The rapid "power on - power off" situation blows the diodes.

But my money's on an inernal short in the alternator.
Not uncommon.

Barry

E200 wagon 1994 110,000 miles.