This is for the blue/black XERUN 150A.
Does anyone know where on the internal board the full input voltage can be taken after the switch?
I feel it would be nontrivial to figure out since the tiny switch is presumably driving some kind of logic gate / solid state switching, not actually switching the full current. I'm hoping someone may have done it already.

Basically I would like the existing switch to also turn off the external BEC, rather than the BEC always staying on (or needing its own switch) when connected directly to the battery.

Thanks!

I was going to try the same thing on the stock esc that came with my Traxxas TRX-4 when I got it last month. I ended up just swapping in a Tekin RX4 and roc412HD brushless motor before I tried. I'm still curious too if anyone has done it before.

As far as I know, there's no way to do this. As you mentioned the switch isn't inline with the main voltage. You'd have to find the BEC circuit on the circuit board and wiring in your external BEC to the input voltage side of the circuit. I just ended up connecting the external bec to the battery and putting a switch inline from it's output. Making sure I had a switch rated for the appropriate amps. I know that in this configuration, the BEC is powered if the battery is plugged in, but I suspect the draw is really low and what I wanted was to make sure my receiver was off.

Appreciate the thoughts.
I was thinking about taking voltage from the input side of the existing BEC circuit, if I was to establish that, would there be any potential pitfalls of using that as my (now switched) input?
I figure there might be some current limitations, given those traces might only be made to cater for the factory 3A, but for a little more info this is on a 1/8 buggy with single servo and RX, and a conscious servo choice (Futaba or Sanwa, where peak current draw seems to be ~2A max at stall) so I think in this particular application that shouldn't be a concern. I really just want the higher voltage since stock is 5.75V.

I'm a little reluctant to go poking around looking for that input voltage, especially if someone else has already figured this out (the ESC has been around for at least 6 years now!). It's a double decker board but I'm pretty sure the BEC reg chip is on the very underside. If I do try to figure it out myself I'll use a minimal power source and perhaps a current limiting device in series, if I find out I'll share here for future reference.

The other relatively neat (and safer) option would be to gut the factory switch and route the actual switch (not setup pushbutton) through a new DPST switch, and use the extra pole to switch the BEC.

Open to further thoughts or suggestions.