Anybody know right off if it's even possible to connect the brake lights on a trailer (or another add-on) to those on the truck itself?
For example if I wanted the lightbar to come on when I have the truck's headlights turned on, instead of just being on when the truck is running, where the lightbar is an add-on? Or the brake lights on a trailer don't light up unless you're applying the brakes on the truck?

it is possible, however it's not possible for them to activate with the switch for the headlights and it hasn't been in the old spintires either, no one knows why.

i do the workaround of them only being on at night (and then having the same addon again with them being always on to give the player a choice).

for them to be on only at night you put the lights under <Ignition> and add SizeMultAtDay="0" to every flare or light you want it to be (or the template if you're making one)

brake lights go inside the <StopSignals> tag, reversing lights go inside the <ReverseSignals> tag

@hydrowasul
That was the conclusion I came to after some experimentation, thanks for clarifying that.