line out converter
 

I'm looking at a PAC AOEM-CHR2 ~ ADD-A-AMPLIFIER ADAPTER LINE OUTPUT CONVERTER AMP HI-LOW and its adds 4 low level outs to my my 4 channel amp but does the the stock radio control its levels with the front back fader or is it just fixed.

As best as I can comprehend the specs on that unit, I believe you'd set it up with the headunits FADE set at '0'. Each of the four channels the line-out is creating can be dialed in to match the input sensitivity of the amplifier. Here's a quote: Once that is setup, when you change the Fade on the headunit, it'll reduce the voltage going to the line-out convertor on the rear channels (assuming that's what you want to do), therefore reducing the strength of the signal its putting out, and reduce the volume on the rear speakers.

I don't think that unit will automatically compensate for a weaker signal. Changing the Fade/Balance is no different that decreasing the volume, you're just specifying which speakers you want less loud...