Just did the same install today..
I looked and looked and looked..finally said sod it, took the rear fender off and flipped it over, drilled the hole right through to where the wire wanted to be, the tail light, with the fender off it was a doddle to get the vampire taps connected. As you have to take the plate off now and again to get to the tail wiring and bulbs, I cut the GIVI cable as shown and cut in a pair of molex connectors. I sealed the backs of the molex connectors with silicone aquarium seal, and weatherproofed the connector with dielectric grease. I tie wrapped the givi side connector to a stand off on the AMOTO plate, and it's all neat, hidden, and doesnt move. Took me a couple of hours all told to do the whole GIVI mounting plate, wiring, and top box. (Made much easier by pulling the fender).