I think there are two problems. First, you're trying to drive an amp looking for much greater pre-amp level than a tablet, with output in millivolts meant for ear buds, can provide. Second, you're paralleling each channel output into two (F/R) channels which drastically cuts the already weak level to each amp input. Have you tried it with the tablet output driving only the F or R amp inputs? You may get enough level, but distortion may also be high with the level cranked high on both the tablet and the Axxess module.
I think you need a pre-amp that converts 2-chnl (L/R) from the tablet into 4-chnl (FL/FR/RL/RR) at a level appropriate for the amp. The Berhinger unit is still two channel. Suggest you call Crutchfield or a car audio specialist who's familiar with the hardware.
