check fuse — remove and replace. Old fuses can look great on the outside but actually be really crappy when removed and inspected. Have a look at my pic in this post.
http://www.peachparts.com/shopforum/showthread.php?t=90715&highlight=fuse
Then check these (in varying combos perhaps):
1 Door needs to be open (no key at first)
2 key in position 1
3 key in position 2
4 Motor running
Try 2-4 with 1
Check the 'convenience fuse' -- not sure where it is located on your model but was a source of several intermittent problems (due to burned up fuse noted in above post) on my 300E — dealer was no help