if you were to gut the housing could you fit one of the new Progressive dynamic converters inside?
they are very easy to hook up, looking at your pictures, the black cable going in appears to be the 120v leads, the blue should be to the
battery, and the red/white are internal to the converter
all the switch does is either connect your load (lights etc) to either the red wire from the converter, or the black wire from the battery(in your case the fused blue wire)
Linear Mode
