I do this all the time using these stick-on cable tie down units. You can run a regular tie wrap through them to secure your wiring. You might be able to adapt this to out-of-sight locations on the towed vehicle and secure these cables. Then adapt a 4-way plug on the two vehicle end and a 4-way socket on the other end to plug your magnetic lights into it. I would not be a big fan of just laying the rear lights cables on top of the towed vehicle...

What I would not want to happen would be the wiring to start flopping around in the wind and doing paint damage to my towed vehicle...
I have seen a more professional wiring done where a separate flip lid cable connector was installed on the front of the towed vehicle out of sight and a separate cable plugged into the tow vehicle. I'm sure that would be your DIODE wiring tied into the running and brake lights on the towed vehicle that you don't want to do...
Roy Ken