1983 Mercedes Benz 300dt cranks but wont start

My car has been having a few issues that maybe are related to it not starting. It has a very rough idle. The vacuum system has issue, the doors don't lock consistently and the car only turns off with the key half the time. There is also a problem with the wire that goes from the positive connection on the batterry to the starter, sometimes I have to pull on it to make it connect and when I get under the car to look at it the threaded cover on the cord in frayed. So, on the fateful morning when my car finally wouldn't start I tried starting it as usual and the car just cranked and cranked and nothing caught. Then I looked out and saw smoke and what smelled like burned electircal wires soming from the area where ther wire connects to the starter. I stopped and came back to it later and it doesn't smoke anymore. When I crank it now it gets slower and slower. Right before it happenned I put in a new guel filter and a new inline pre-filter.