urbanjo,
Wiring a toad is much simpler than you think. I'm not a fan of using magnetic lights and having the cord drape all over the place or, the process of drilling holes in the tail light housings and adding auxiliary sockets and bulbs. Lots of folks do it but, it's just not for me. I've wired all my toads, a Toyota pickup, (7) different Jeep Wranglers, a 2011 Honda CRV and our present toad, a 2011 GMC Sierra 4x4 Extended Cab truck ALL THE EXACT SAME WAY. And that way was/is to "T" into the wiring very close to the tail light, solder everything up nicely and efficiently, then add the correct Radio Shack diodes and tape and secure everything all up.
Never had any wiring issues for over 25 years of towing and, the stock toad running, brake and turn lights have always worked flawlessly. When done, your toads done for good. No goofing around with removing and storing those magnetic lights and cords (which accumulate road debris, dirt and mud etc. and then put it back onto our cars surface when you go to put all them back on again).
You say you've done lots of work around the house, well then this is the same thing. If you'd like, I can send you a simple wiring diagram I drew up for the CRV which should, be the same for the Fit but, not sure. Good luck.
Scott