These are used to run the fans at slow speed.
I've had to replace at least one on my Boxster. I had the tech replace the fan and motor assembly.
The resistors can take a beating. Often what I'm told the techs find is if anyone had been in this area the ballast resistors are not mounted properly but just shoved/jammed between some plastic panels. The resistors need to be properly secured so they do not run too hot and are not subjected to water splash which can damage them. Even if they are mounted correctly they can go bad over time.
If there is a way to salvage the radiator fan motor and reuse this (or in your car's case both of these) by replacing just the resistors I'm sure someone has done this and written about it. Otherwise you are probably looking at replacing both radiator fan motor assemblies. I seem to recall the cost of one of these was not that much higher than the cost of the resistor so if this is still true (assuming my memory is right about this) then with new radiator fan assemblies you get not only new resistors but new fan motors as well.
(My Boxster has been through one and my Turbo has had both radiator fan motors replaced. One due to a broken shaft, the other side because while the fan was running it was not moving nearly as much air and was using way more current than the other side.)