as long as the "wire back to the starter" goes to the solenoid, that sounds right .... grounding the switch case shouldn't matter, having a bad ground on the starter could make it draw double or triple the current it normally would, or even make it 'find' a ground via the switch case, so yes, that'd be the first fix, it's necessary to have excellent connections on all grounds, very clean attachment points, and preferably externally toothed star washers to dig in to both the cable end and the grounding metal
Bill
