The logical place is in the tail where the loom feeds the rear light cluster. The non-invasive approach would be to make up plug-and-play Y-harnesses using the appropriate connectors, one wee harness for each lamp. On the light bar's bundle of wires you'd feed each into the corresponding connector.
Or, you could cut the bundle before it goes to the cluster and add a 6-way plug and socket (e.g. the Hitachi kind, presuming one redundant connector hole) into which you could then insert your own simple 6-way Y-harness (one end feeding the cluster, the other, the light bar). If you chose to remove the light bar later then you'd just reconnect the 6-way.
Or, you could splice in extra wires directly. The nice way to do that is to warm the insulation, cut and peel it back and solder on the extra wires, then wrap with gently heat gunned electrical tape.
On my bike, the previous-but-one owner had installed a towbar, and for the lighting he had cut each of the wires to the lamps and added each additional wire by twisting the strands together and wrapping them in tape. No solder, not even Posi-taps. When he sold the bike without the towbar he just snipped those extra wires off short and left the twisted connections. After I discovered the mess I think I soldered them, and wrapped the snipped ends for later projects.