The item that generally needs replacement is as follows:


http://www.allpartsexpress.com/ShopByVehicle.epc?q=1984-MERCEDES--BENZ-300dt--/--L5--998cc--3.0l--Diesel--Fi--Turbo-Brake--Components--Wheels&yearid=1984%40%401984&makeid=MERCEDES+BENZ%40%40MERCEDES+BENZ%40%40X&engineid=1193789%40%40300DT++%2F+L5-2998cc+3.0L+DIESEL+FI++Turbo%40%40300DT&catid=Brake+Components+%26+Wheels%40%40Brake+Components+%26+Wheels&subcatid=Front+Brake+Pad+Wear+Sensor@@Front+Brake+Pad+Wear+Sensor&mode=PA

The part that you are referring to is the cable that begins inside the firewall and ends at the brake caliper. This cable eventually suffers from damage to it's insulation, however you might get by with a good application of friction tape. I recently replaced these on the SD, however, most just use the original part.

FYI, the low limit on the thickness for the front rotors is .764"