This is OEM. It was an option on the Wildcats in 2011. It was supposed to be an upgrade, but the general public felt otherwise. For us, it has been really good. It is really nice to never run out of hot water with 4 people taking showers. We never had any problems at all until this issue due to me not getting all the water out. Montana is much less forgiving than Houston, TX.
So I pulled one piece of panelling off, then cut a hole in the next panelling behind that, and got to the fittings and back of the unit. It must have been assembled prior to all the skin being put on.
Thanks for the help. Any advice on putting the new one back in will be much appreciated.