My understanding of the trutrak autotrim is that when the AP is off, the signal from the manual trim switch to the trim motor is direct so a failure of the auto trim will not have an effect on the trim system. I also talked to Zach at Trutrak and he says that the relays they use and the system design is set to fail to manual trim.
>> up to here I agree with you. The safety trim box would protect you from any failures in your manual switch system which are connected when the auto-trim is off.....if the auto-trim relays return (via spring) to the unpowered position.
It does not follow that safety trim does anything whatsoever if there is a problem with the AP auto-trim function.
So it seems unlikely that the AP would be involved in a runaway trim scenario and that the safety trim box would indeed protect against that.
Dan