I had Bob's Speedometer do the rebuild. They did an excellent job.
https://www.bobsspeedometer.com/Yes, the S is for the sending unit. You send 12v to the gauge unit at the + terminal, then the S goes to the sending unit. The sending unit is a variable resistance, so it changes the current through the gauge to deflect the needle. You have to make sure you have a good ground on the sending unit side.
The plugs are just upgraded light sockets for the dash lights.
