I don’t have the Multi Flex gate, so not sure on the mounting, but 23 years of car audio experience, great choice on the Focal amp, always quality power out of them. I’d say easiest way would be to just go a basic toggle switch for remote turn on off of a 12 volt constant feed. If you don’t want to chase a feed off the fuse box, just run straight out of the battery input of the amp into a toggle switch. I’d fuse it to be safe, 7.5-10amps should be fine. That way you don’t have to leave the ignition on and can just play the tailgate set up.