It's 15mm. I bought a combo spanner and heated the open ended end 90° as close to the jaws as possible, this did the trick. You might also like to try a 3/8 drive crows foot spanner. It's tight though !

__________________
1993 190E 2.0L (Euro.) sold
1991. 500 SEL (Euro.)
1991. 300 CE (Euro.) sold
1993. 500 E (U.S. spec.)