That's what your meter is for.  Where does this 5V exist, and what do you
have to disconnect to stop it?  How much current is passing through the
bulbs?  Does the voltage change to 12V when you remove them?  What's the
voltage on the ground plate?

LEDs present additional opportunities for confusion for two reasons.
First, their operation is backwards to incandescent lamps, where you have
to heat a metal filament to glowing point before anything visible happens.
With an LED, once you reach the necessary voltage of roughly two volts per
series LED, the LED will light up visibly even if only microamps of current
are passing (of course it will be very dim with a microamp or two, but it
will continue to brighten as available current goes up).

Second, LEDs themselves are polarity sensitive and only light up in one
direction.  Many LED lamp assemblies however contain a diode bridge that
steers current in the correct direction regardless of polarity.  It's quite
possible to have both types in a system, which can make troubleshooting
more interesting.

Yrs,
d

On Tue, Apr 11, 2017 at 4:31 PM, Stuart MacMillan <stuartmacm@gmail.com>
wrote:

>
>
> What confuses me is there should be no power going to the tail or brake
> lights with the ignition on until you turn them on or step on the brakes,
> so where is the constant 5v back feed coming from?
>
>
>
> Stuart
>
>
>
>
>