I have been running JL Audio for a long time and they are one of the best.
I will be using this one when i do the stereo;
FiX-86
OEM Integration DSP with Automatic Time Correction and Digital EQ. 8-ch. Inputs / 4-ch. Stereo + 2 Subwoofer RCA Outputs
The FiX™ 86 is a state-of-the-art digital signal processor engineered to deliver outstanding sound from factory source units, while retaining OEM fader control. Unlike typical OEM interface products, the FiX™ 86 automatically corrects time-delayed factory outputs prior to signal summing and EQ correction. Calibration takes just a single button press and about 30 seconds to complete, with no special equipment needed.
With a powerful 24-bit DSP processor and quad 1/3 octave equalizers onboard, complex factory signals, up to 3-way front and 2-way rear, are combined and level matched automatically into six, full-range stereo signals: Front (fader-controlled), Rear (fader-controlled) and Subwoofer (non-fading, front+rear summed) outputs. Differential-balanced inputs accept virtually any OEM analog audio signal, from low-voltage, line-level to highpower, amplified speaker-level outputs (up to 30V RMS).
Activation methods include 12-volt or Auto Turn-On (Signal/DC-Offset Sensing) to trigger a dedicated remote turn-on output for aftermarket equipment. An optional DRC-100 or DRC-200 Digital Remote Controller (sold separately) adds Master Volume Control, Subwoofer Level Control and preserves hands-free calling features, all from the driver’s seat.
Achieving great sound from your factory stereo has never been easier, or sounded better... Now with fader retention and subwoofer output!