I think your going to have to use all 4 inputs on your amp. I would use front rca outputs from your radio if you're using your amp for the front speakers. this will allow you to keep the fader option. you will need to buy 2 y cables which makes you're front rca set to 2 rca sets & then wire left channel to 1 & 2 rca input channels & wire right rca to 3 &4 rca input channels. this will allow you to feed a stereo signal to your speakers.
Anybody can correct me if i'm wrong, but this is how i think you do it.
I don't know myself, but I found this in some dark corner of the internet:
"[2 channel] amplifiers running in bridged mode accept a single input signal. Normally, the input will be "Channel 1" or "left". Manufacturers vary however, so check the instruction manual for the proper input wiring procedures."
Found this while searching for a cached(sp) page of Xtants FAQ:
How to supply input signal to a 4180c/x & 604x when bridging both front and rear channels for high powered l/r operation
A common method of creating high power for a left 4 ohm speaker and right 4 ohm speaker is to bridge both the front and rear channels of any Xtant 4 channel amp. The resulting 2 channels will provide double the power to each 4 ohm speaker. The input jumper must be in the 4 channel position. Use an RCA splitter cable in front of the L/R input cable to create both a left and right high powered channel.
The Xtant web site was not working when I found this but it seems to work now.
Just as an update for everyone, I just got done speaking w/tech support at xtant. The person I spoke w/verified that I do not need to use both sets of rca inputs. He stated to set the jumper to 2-channel mode, and just use the front inputs on the amp.