I recently changed the car - and am about to re- fit the tow electrcs, as my instructons ("Do NOT fit a bypass relay") were either ignored or the guy at the garage hasn't a chufin' clue. On the plus side he did listen when I said "NO scotchlocks"......
Bypass relay - does exactly what it says on the tin - bypasses the can bus. It hooks into the lighting wires like back in the day, but instead of the bit you hook in going straight to the socket & confusing the lighting module with extra bulbs & stuff, it goes to feed some incredibly high resistance, low current triggers in a box of electronic gubbinry (which fools the can bus into not noticing it, but detects when voltage is applied to the lamps).
The box of electronic gubbinry then essentially fires an electronic relay which is fed form an independant source to the lights (hence needing the +12V power) and switches the trailer lights by "remote control" form the car's lighting circuits.
Thje black box should also have the annoying buzzer (to tell you the trailer indicators are working) and the "car fog off" functionality built in.
All stricly legal WRT the lights, BUT........
- doesn't tell the ABS you have a trailer on the back
- doesn't tell any stability modules you have a trailer on the back
- minor issue but bl**dy annoyingly won't switch off rear park sensors when you have a trailer on the back....
And that is why you really should get a CAN talking one (if your car supports it) as I certainly don't want to get into a "non linear braking situation" and have a module confused midway through correcting a skid by a 3/4 ton shove from the rear.
I got a Westfalia CAN kit -(google "PF Jones") Entirely plug & play (no cutting or splicing wires). Some cars auto detect the new module, Ford unfortunately don't so once fitted I'll either need to take it to a dealer to activate the module or work out how to get ELMconfig to run reliably on my laptop...... More of that anon as I still need to remove half my boot trim to fit the kit and It keeps raining when I'm free!