The problem ( usually ) when you tap a starter and it starts to work isn't in the solonoid but the motor it self. What happens is when the brushes ( the part where the elctrical connection is made to the rotating porton of the motor the commutator) start to wear out. They are pushed down by a set of springs, and when they wear out the spring can't push them far enough to make contact with the commutator. So the long and short is that the motor can be rebuilt by taking it apart and replacing the bearings, seals and brushes.. or just get a new one. I have rebuilt these in the past and it's never as simple as you think. As said above the ground is provided by the but connection to the block, and there is only 2 wires going to the starter ( both positive ) and they should be kept clean to keep resistance down at that point.
Hope this helps.. I'm new here