well, you can always add an RCA converter and run RCAs off the factory deck, tap the 12V power line for signal and run power wires just like any other sub.
Where you may have problems is with the DSP, if so equipped. But the sub would play, but might not sounds well with the factory stuff.
<span style='color:gray'><span style='font-family:Courier'>1995 e36 3 Series RIP
1999 e46 3 Series BURN IN HELL
2000 MR-S Roadster= Current</span></span>