Mercedes Benz repair: Electrical/ Starting/keeping battery charged, cigar lighters, window switches

Question
I have a '79 280SE Mercedes. I have to jump start it each time I use it. The battery has been replaced. Problem was resolved for about a month. This vehicle gets little use. I ran a meter across the battery terminals. Got a reading of 7.8 v. Jump started the car and let it run for a few minutes.I am geting a reading of 13.7v off the battery terminals and the same reading from the + terminal to the engine block. I've put  test leads on battery ( - ) terminal and engine block and get approximately  0.02v reading. Where do I go from here? I would like to repair vehicle if economically practical. I live ln Brooklyn NY.
Any help you can give me is appreciated. Thank You  - Robert Kent

Answer
There is a circuit open and it is drawing juice from the battery. Typical are window switches, Cigar lighters, Dome lights, Bad ignition swithces, Trunk lights, etc. Try disconnecting a fuse to a particular group and see what happens. Process of elimination.