It might have been me who suggested using two SPDT relays to take the load off the switches for the power windows. If I recall correctly, the yellow wire in the switch harness is a direct +12v source comming from the window switch relay (power only on when ignition switch is on), and then going to a circuit breaker that gets power from the ammeter connection in the dash.
If you tap (splice) into that yellow wire for power you won't need to add any new wires.
The rest if the connections are just made to the factory motor connector. Seperate the factory connector going to the motor, and the side that goes to the switches is now used to energize each side if the relay, the connector wires going to the motor now connect the the high current part of the relay (the common side.) On the SPDT relay, one side is connected to the +12v power from the yellow wire, the other throw is connected to ground. Each relay will have two ground connections one for the relay coil and the other to groung the wingow motor through the relay. The idea is that both normally closed contacts of the relay(s) are grounded, so both wires to the motor are grounded when no window switch is pressed. When you press the window switch, you only apply +12 volts to one relay at a time for up or down because when the 12 volts energizes one relay, the relay switch breaks its ground contact and switches the the +12 volts supplied from the yellow wire, while the other relay stays grounded.
The relays will need to be grounded to the door, and my door seems to get a good ground through the hinges, but you could add a ground wire if your having trouble getting a ground.

Anyhow, I hope that helps explain the workings. It looks like you have a good idea of what to do, and that connector looks really nice. If you need more help I could draw up a schematic with the realy terminals identified.