.... I think the system is just measuring the impedance. This is why adding more stuff can trigger messages or faults. I am aware some devices have additional circuitry in them to report more intelligent responses than "bad", but I don't know which ones or exactly how.
To be completely honest, it's these unknowns that prompted me to just use these anytime I am tapping into a source wire. It's my way of getting the signal and doing my best to leave the CANBus stuff alone.
Another full disclosure: If the system was measuring current it could still have issues with the added relay. This said, I have used dozens of these and never seen this. It has fixed all CANBus related flickers, chatters, buzzes, and messages to date.
Fair enough, and thanks for taking the time to explain.
I will tap into the high beam wire for the relay, as many other members here have done - with no known (to me) problems - to trigger a relay for added lights. I will put my ear to the relay and watch closely for any sign of... whatever abnormal. And if I notice anything, expect a call from me
FWIW, found this info on the type of current used in the headlamp circuits:
The headlamp system includes the IC, the Steering Control Module (SCM), the BCM, the headlamp switch
and the multi-function switch on the steering column. The headlamp bulbs have a path to ground at all times
through their connection to the FEM (Front End Module) wire harness. .... The BCM will store a Diagnostic
Trouble Code (DTC) for any shorts or opens in the headlamp circuits.
The BCM monitors a hard wired multiplex input to determine the status of the headlamp switch. The BCM
also monitors electronic headlamp beam select switch status messages received from the SCM over the
CAN data bus. The BCM responds to these inputs by providing a pulse width modulated voltage output to
the headlamp bulbs
through high side drivers on the right and left headlamp feed circuits to illuminate the
headlamps, and controls the high and low beam shutter operation
through high side drivers on the right and
left high beam feed circuits. The BCM also sends the appropriate electronic headlamp beam status
messages to the IC to control the illumination of the high beam indicator.