Ignition fuse location

[Strider199]

So I am helping my Dad install a hydrogen system in his 2008 Yaris and he does not have his manual. So checking on here and my 2012 yaris, the fuse looks to be a 7.5 Amp located on left side fuse holder box under the hood. When I checked the two 7.5 fuses there both had power to them when the engine was turned off. That wont do for a hydrogen system so my question is;
is there a spot to hook up a lead to where the voltage is 0 when the engine isn't running?
I have installed a manual switch which will power down the hydrogen unit but if you forget to toggle the switch after you turn off the engine, BOOM goes your in-take manifold after an hour as the hydrogen keep producing until it detonates for the heat in your head. Yes this has happened in a previous DIY.