Savage XL conv. problem with Reverse module

Ok, here is what I do:

Ch1 is of course steering. Ch2 throttle. Only forward makes sense (on the ESC I enabled only forward mode). The brake servo is using Ch3! So it is mixed with Ch2 on the transmitter. In the car, the HPI mixer 'listens' to the brake servo (or ESC signal) and activates the reverse servo when the brake has been activated for 3 seconds.

So, it should only be able to switch to reverse after full stop - brake for 3 seconds, not just when doing releasing the throttle or doing a quick brake.

Can I wire it differently?