you have to watch with mth engines when you test voltages and are unsure of the ground . if you measure to the wrong ground you can blow the boards ! metal chassis ground Is not the ground used! PV is the voltage created for smoke n lights but not related to actual ground, purple wire is where pv (positive voltage) is created!
as far as the led your talking about the 220 ohm is in series with the (led) you could even run your led from track voltage with the 220 ohm resister in series!