I am planning on running line output converters from my 5th Gen Non-Bose HU to my 5-Channel Alpine amp in my trunk. Do you think that I would have significant signal loss if I ran the 4 speaker wires from my amp back up to behind my HU and spliced into the current factory speaker wires? I would probably use 12-14 gauge oxygen free speaker wire. Let me know what you think. Thanks.

 

That's how I wired mine in and it sounds fine. Although it will sound so much better with a good quality replacement headunit. I plan to replace mine soon!

 

99% of the people in the world wouldn't be able to hear a difference between that and running new wires straight from the amp.
the other 1% will only hear the difference because they know how you wired it and think there's too much cable in the line.

At low powers and lengths like you're dealing with in the car, it won't be an issue. Now if you were running several hundred feet of cable with several hundred watts of power, THEN you've got line loss issues.