The Valkyrie does not have an OEM modulator. If you have one a previous owner installed it. Having said that, LED's draw very little power - that is, almost nil - a small fraction of the load of a tungston bulb. Bet ya a beer you can add that LED string to either the modulator output or to the stock brake light circuit and neither one will "know" it's there.
Yep minimal load, I have a large number o LED's attached to the brake light circuit directly and no issues so far other than being WAY overly bright and annoying to anyone behind me. Seems like there are either 18 or 24 LED's under each saddle bag and another 35 or so under the trunk ad probably better than a dozen on the wing mounted on top of the trunk...

If memory serves I split the wires under the right side of the seat and spliced them there between the fender and the side rail that the trunk sits on. Ran the wire along the inside of the saddle bag stays up to the seat area and the light bar under the trunk just ran the wires under the trunk to the side of fender under the seat,,,, easy peasy!
Since then I have also added the wing on the top of the trunk and it also has a set of LED's all the way across the underside of it....