iirc that relay needs to be grounded - least it is on the 550s I set back up last weekend.

That'd leave 3 posts on it unless it's a 5 post relay. Then one wire would come in from the battery to the relay. This should be a gauge thicker than the rest of them and fused.

Another - probably doubled up, 2 wires into a single blade in other words - would run out to the lights.

The 4th should run in from the switch.

At the switch, 1 wire goes to ground. A second is that one going out to the relay and the third comes in from the circuit you tap (I used the parking lights).