I have been working on harness training Kiwi as I do not feel safe taking him out on our new upper floor balcony. "Mr. Amy"

is right, just take it slow and go at Bonkers pace. I chose the feather tether, which goes on differently than the aviator. We've had Kiwi a long time and have that trust built and Kiwi is pretty fearless about new things, so he was not really apprehensive of the harness until I started trying to buckle it on. If Bonkers is frightened of the sight of it though, you will need to desensitize him. Just keep on putting it as close as you can to him and rewarding him the closer he lets it get until you can touch him with it. You may even want to try holding highly desirable treats close to the harness so he is tempted to approach the harness to get the treat, once he's a bit less frightened of it. Parrots explore with their beaks, so once Bonkers isn't deathly afraid of it, allowing him to nibble it a little will help him understand it isn't harmful.Once he is comfortable being around the harness, start touching him with it while you reward and praise him. Once he's comfortable being touched with it, then move on to whatever the first step is to put the aviator on (I think with that one, you have to entice them to put their head through the hole, which is why I personally didn't go with it).
Progress has been slow for us (as it often is teaching Kiwi new concepts he's not overly interested in), but I can now get the neck buckle on without him freaking out. I only leave it on long enough to give him a couple treats then take it back off, so he associates the harness going on with positive things and understands it comes off and won't be on him permanently. My husband (who is Kiwi's "favored" person) has managed to put the harness fully on him a couple times by taking him in a dimly lit room where he's calmer (of which he has been rewarded with extra special treats, like little bits of cheese

while it was on), but I would definitely not recommend my hubby's "techniques" with such a new bird. Hubby and I often work at different rates of success with Kiwi due to him being more tolerant of what hubby does to him, so I'm still working on getting him to be ok with me putting it under his wings. Parrots are funny creatures like that.
I think, given enough time, you should be able to work with Bonkers and get him to wear the harness. The time thing is really your biggest hurdle, having the patience to see the training through even if Bonkers decides to go at a snails pace. The companies that make these products show trained birds in their videos. It makes owners of untrained and apprehensive birds feel like it will never work for them because their bird is still not wearing it after a month or 2 months or even 3 months of barely any progress, so the owner never sees it through and the harness gets stuck in a junk drawer somewhere. One thing Kiwi has taught me is PATIENCE. Every bird can be trained, it's a matter of how much time YOU are willing to invest in that particular type of training
