My 93 Ranger will not start. (XL model with 2.3L and 5 speed manual)

It turns but very slowly, almost as if the battery is dead or dying. I`ve changed out the starter, the hot primary battery cable and used a fresh battery, even pulled the plugs on one side and squirted a little WD-40 in then tried w/out the plugs to release the compression drag but it still turns very slowly and the battery wire gets hot. There is some corrosion visible on the wiring and I can see some bulging under the insulation near the ends of some of the wires in the starter circuit-could it be a bad wire due to corrosion causing resistance? Its a nothern car and has seen plenty of winters and loads of road salt. Is it difficult to replace the starter wiring circuit? A neighbor suggested by-passing the starter cicuit w/co-start switch but how tuff would that be?