Exactly as described by turboed13b.
"Run +12" meant "connect a wire which provides 12 volts". As turboed13d said, tap any black wire in the harness for this.
Automotive-style relays have standard pinout numbering. The trigger coil is the electromagnet inside the relay that controls the switched circuit (terminals 85 and 86). The switched circuit is just what is says on the tin, the circuit that is opened/closed by the trigger coil (terminals 30 and 87).
When the neutral switch closes (in neutral) this will energize the trigger coil which will close the switched circuit, allowing 12 volts to flow from the harness into the new neutral light.