If you have an ABs model, I would use a trailer wiring isolator harness and come off of that. Or if you already have one installed for a trailer, tap into that. Yes I am paranoid about tieing into factory wiring 4:
Honda has downsized the Wire Guage too much to trust adding any load to the OEM harness.
Most Isolator's have Plug-n-Play connectors that attach to the Tail, Stop & Turn in the area under the trunk behind the "Fender". From there you can run the output wires forward for your Sidecar harness and Solder as needed.
The ShowChrome Isolators are cheep enough and small enough that I used two on my Rig. One feeds the Sidecar and the other feeds that Trailer.
Not having any max capacity info to go by I went with the dual units to be on the safe side.
If you want to use only one Isolator for both Sidecar and Trailer lights be sure it is heavy duty enough to run them all.
Scotch Locks and other quick connect Wire Taps should be outlawed IMHO.
Also, if you need a fuse block I highly recommend the PC-8 from Eastern Beaver. They also sell the Posi-Locks, and other assorted electrical items. Service was quick and if you have any questions you can email him. He is in Japan so you might not want to phone him. http://easternbeaver.com/Main/Products/Fuseboxes/PC-8/pc-8.html