Your cars Engine Management Sytem has not been working , either due to wire harness or OVP voltage supply interruptions..[ or both]...so, the cat gets soaked and the rear 02 sensors job is to monitors the cats output. It fails the test and pops the code.... when the engine management is to spec , the cat will burn off the remaining crap and return to a normal specs.

This can also be accomplished by a hard run , under load [ read hill] AFTER the cause/s have been rectified .. [ with the OVP being the FIRST suspect]

It is quite common to see many parts changed before the OVP gets even looked at. With their notorious fail rate, this has always amazed me .. Specially when the owners have used a Benz Dealer .. just too hard for me to comprehend how this can possibly happen........but if you do a search, one will see it happens consistantly. I would venture to say that the OVP has caused more guys $2K due to mis-diagnosis than any other simple $60 , plug-in part.
I am one of those old school techs who, when we percieve an electrical power feed malfuntion, start at the front of the feed line [ Battery supply voltage] and work foward. The next stop , of course, is the OVP.. Now , I guess we could start at the other end [ The ECU] and work backwards to the voltage supply, but I am just lazy and want to find the problem as soon as possible.........meaning I am not as concerned about racking up the bill as I am at solving the problem.
Any good tech dept not only knows of OVP problems, but also has a known good OVP in their toolbox for rapid check/verification.