It's one of the front two seatbelt buckles. Check them closely, see if they are damaged in any way. Also, it could be damaged from a liquid getting in there.
If you really want to know, you'll have to take it to the dealer to have them plug your car into their scanner (not OBD, a different plug) to see exactly which one it is.
The SRS light would come on in my car for about 90 seconds every time I started the car, and then go out. About the same time, I noticed my driver's side belt buckle was somewhat separated. I replaced the belt buckle myself (part cost about $75 for me), but still had to take it into to the Mercedes dealer to have the code reset!
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!)