I do have a wiring diagram and have plugged the LEDs into the right wires, or so I think I have. The problem is that I get no voltage coming out of the brown/white wire. I only get that LED to flash when the blinker is turned on or the key is switched to "P".
This sounds a bit like a mis-conneciton in the headlight? But first - which 550 do you have? The early ones use the keyswitch under the gas tank, front of frame. The later ones use the key up by the instruments. These main harnesses are different, as are the keyswitch connections, and some imaginative wiring changes have to happen to mesh the 2 different types. The handlebars switches are a little different, too, which may be where the issue starts, here. The vendors often don't know the difference, with 4into1 being the least aware of which parts fits which bikes, in my experience with them. If it wasn't electrical, they can be good about exchanging stuff for the correct part when they mess up, though, so they aren't "bad" vendors, just confused sometimes.
The Running Lights and the Parking Lights are the same circuit, the Brown-White wire stuff. These are also on their own fuse in the fusebox.
The Turn Signals are powered via the Black wire system: it reaches the turn lamps via the flasher when the TRUN switch is in either position.
