Towbar Wiring

I've seen the guide to wiring the towbar using the connection near the wheel arch, beneath the panel that you access from inside the cabin. I've built a storage unit and platform in the back of my car though, so need to connect the wires somewhere else. Can anyone shed any light on my options here? I've heard something about using the wires under the nearside rear tail light cluster.

Cheers mate

Was it an easy job? This'll be my first attempt at wiring

You will still need to access somewhere inside to place the indicator buzzer.

Was thinking that I'll just mount the buzzer somewhere discreet on the plywood structure in the back, but then that would lead to a messy cable running on the outside and passing in via the door, so won't work. Looks like I'm going to have to strip the ply out and take the panels off after all...