So I have been told the RCA from my head unit for my sub is shagged, I get heaps of hum and its nowhere near clear.

On my head unit, I have both high and low out puts and currently I have the front speakers hooked up to the high outputs and the rears/sub through RCA to the amp.

So I was wondering if its possible to run rear L & R RCA into the standard LTD high series AMP on channels 1 and 2 and then bridge those channels and hook up to my sub?...I can use the high output from the head unit for the rear speakers as I think they will be plenty loud enough.

Least then I still have the sub working (kinda) and the fader will still work as it should

When you get this hum it is normally from a damaged earth on the RCA
Solder some speaker wire To the terminal and earth it. Should fix you're problem.
Could also be from running RCA too close to the power cable.