I used to fix car audio gear about 10 years ago. When having a stacker was just THE BEST! :D
From memory in the stacker cable there were (along with others) 4 audio wires L-+ and R-+. So if you picked up those four wires from the cable and tricked the headunit into thinking a stacker was connected (otherwise your headunit would just skip the stacker option) then I can't see why you couldn't use that as an input.
Start tinkering!

