I am quite new to this handyman stuff but I've been relatively successful on fixing stuff around the house recently. But I'm stuck on this. I need to access the shower cartridge. But the bonnet (piece K in the manual, see pics) is in the way, and it appears I should be able to just screw it right off. But I can't get my hands in there well enough to get a good grip on it. If I use pliers or channel locks then it crushes the threads on it. It's threaded all the way around.

I'm doing this because based on what I found online, my shower cartridge might need cleaning/changing out. Our on demand water heater will send hot water literally everywhere in the home except the shower head. The tub faucet is fine too. If I turn on the bathroom sink's hot water, \*then\* the shower will receive hot water fine. I'd like to at least look at the shower cartridge and discern whether it is faulty or not. 

Is there a way to get this off without having to cut a bigger hole? Am I going about this the right way? It's strange to me that the hole would be made too small to do work on the area.

Here is a link to the manual: [https://www.manualslib.com/manual/3760488/Allen-Plus-Roth-873x-567001.html#manual](https://www.manualslib.com/manual/3760488/Allen-Plus-Roth-873x-567001.html#manual)

Thank you for any advice or suggestions!

How to get this stainless steel piece off?