They hooked something up wrong, plain and simple. There's no reason, other than an improper installation that would cause that issue. If you only have one set of RCA jacks on your stereo and they used a Y adapter to split the source, that could potentially throw off the sound, but it would likely be that you'd lose sound if you adjusted the fade control to the front or rear, since most aftermarket stereos with one set of RCA's is either a subwoofer, or rear output. You'd still have balance control from left to right.