That does look like the wire to the brake pad sensor. The sensor is directly on the brake pads, and it looks like this:

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1CQ0J3JZ41W71F6MWU&year=1981&make=MB&model=300-TDT-001&category=All&part=Brake+Pad+Sensor