losing sub volume

I need help!  I installed a Sony  ZR 1252 amp (125 RMS per channel.) on 1 (one) alpine 1043D 10 Inch sub in a .65 cuft. sealed box for my gf.  its in the trunk of a 06 pontiac G. facing the rear of the car.  I dont like doing this, but used a speaker out put to rca line in converter to go from the rear speakers to the line/in converter to the amp.  everything sounds ok at low volumes.  however, as i turn up the voulme past mid-way the sub sounds like it is acutally losing volume.  it does not cut out. it just gets softer.  could this be from the line out converter or something in the factory radio that cuts bass to the rear speakers as the volume goes up.  its got me perpelexed.  I do have both the right and left rear factory speakers connected to the line in converter.  someone told me to try just using the left speaker. i havent tried that yet.

I have the dual voice coil sub.  i followed the directions  on wiring the sub and have checked it with an meter. 2.1 ohms.  i am using the bridge terminals on the amp and it is 2 ohm stable.. Ive triple checked every connection.  i sure nothing is out of phase unless the color coding on the rear speakers is not correct. im using dark blue and brown as the positive speaker wires for the rear speakers. If i had 2 woofers i would say it sounds out of phase. any suggestions would be welcome, other than trashing the deck and going after market.  she wants to keep the factory head unit.

If you have a dual 4 ohm driver, your choices are 2Ohm bridged or 8Ohm bridged.  A 4Ohm stereo load is the same exact electrical load as an 8Ohm mono load.  So a 4Ohm stereo load will be the same as an 8Ohm mono load.