I had an issue with my radio not as bad as this one but it may help some.
It ended up being a bad tweeter on the door pod. there was some sort of issue in the speaker itself that started to cause the whole left channel to fade out. Mostly the tweeter didnt work. I finally got a replacement at a car recycle yard. It helps if you have a multimeter you can test the resistance of the speaker coils to see if you have a bad one.
Also, remove and replace the connectors a few times on the back of the headunit and RCU
if you have not and see if that helps.
I thought I was going nuts with mine. I would pull the door pannel off and check the connection to the tweeter.. drive around and everything would be fine. Then I would put the panel back on and the next day the tweeter would not work again. I even cleaned the contacts with some sandpaper and same thing. Not sure what happened to it but I guess it decided to die that week.
I'm not sure if this was related to the tap issue I was having where the left channel was low output. It seems to work fine now although I think sometimes it is still not all there but it may be the tape.
I did find a Mach RCU
at the yard and installed that.. but the gain was too high and where normally i would listen say.. at the half mark, it would be way down with only a few dots to be as loud. Also pickud up a lot more cell phone interference that the stock (as well as ipod harddrive noise) That alone would have made me take it back but it also had an issue with one of the channels sorta fading in and out with heavy bass..
Needless to say I took that back and am using the regular RCU
still. Works fine for me.