[tsubaki]

Not knowing what the bilge area looks like in a "79, I would have to assume there is room to mount the switch out of the way and be close to the batterys. Follow the instructions on the perticular switch you got. Normally all the grounds (-) connect together, the easiest method is battery 1 to battery 2 to boat ground (-). Then on the switch connect battery 1 (+) to battery 1 terminal, battery 2 (+) to battery 2 terminal and boat power to load terminal. If this don't sound right I'll have to get a diagram of the perticular switch you got to definately, correctly name the terminals.