I think I might have a solution to your problem. Ever since you posted your question... I kept thinking that it was a 15amp fuse. Well I finally checked today and sure enough, I have a 15 amp fuse in my M104 motor in 1995 C280. I cannot be sure what the 300E's have though... but this might be a start?
300E owners... what is the amperage in your fuses?
2006 E350 w/ 155k miles (Daily Driver)
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)