On the rear fender lights on the Magnum I think you should be able to mount the ballasts under the fender, you will just have to drill an additional 1" hole to run the wiring through since the factory lights run the wire through the mounting bolt.  There is a second panel on the bottom of the fender and I think there is room between it and the plastic to mount the ballast.  I have the kits to do the top cab lights on our MX 240 but haven't got them in yet.