I just recently set up my MINI Cooper as a toad. I ran the wire for the breakaway switch through the firewall at a point where a main wiring harness passes through. Depending on the car you drive, it should be a fairly easy task of finding such a pass-through point. Look for other wiring, maybe a radio antenna lead, or perhaps even a speedometer cable (not too many cars have these anymore!).
Good luck! Let us know how it works out.
Warren and Debbie, Deep in The Heart of Texas
2018 Winnebago View 24D
2014 Tiffin Breeze 32BR, 2012 Winnebago Navion 24G, 2006 Winnebago View 23H