Seat Belts

I have a 300SL 1991 and have had the seat belt cut, so I need to replace it. I took the seat apart and got to the seat belt retractor, however it's like nothing I've ever seen before. It has the spool of seat belt webbing, and in addition a whole lot of wires, and also a metal tube hooked to a large aircraft type cable. Question, does the tube contain some sort of explosive set to go off when the air bags fire? Can I just replace the belt webbing, or do I need a new complete assembly; and if so where can I find one? I've searched the web and can't find anything like this. Thanks for any help in advance. Ron