I have two toads, my Focus has completely separate tail/brake lights installed in the Ford rear lights, so there is no direct connection between the MH and Focus factory lighting system. But on my Explorer I do have Roadmaster diodes to operate the Ford lights, it has been 100% reliable. Have had 2 prior toads with diodes and never had a single issue with either one. Installed everything myself so I know it was done securely and correctly.
My Explorer has to be towed with the keyless ignition in the "Accessory" mode which means there is continuous power consumption. Add in the Roadmaster EvenBrake supplemental brake system power draw, and a charge line becomes mandatory. While the Focus is towed with all power off, hilly country tends to use the EvenBrake quite a bit, so again a charge line means never having to worry about it starting at the end of the driving day or the EvenBrake shutting down en route due to low voltage.
Just wanted to point out that a charge line can give you peace of mind that your toad battery will be charged at the conclusion of your driving day.
2014 Seneca 37TS
Toads: 2014 Ford Explorer Sport or 2013 Ford Focus ST