if anything get a hi-low converter.
Tap that from the rear speakers which will gain you RCA output.
Connect that output to a mono amp ( 1 channel amp ) to a subwoofer.
You can even pick up one of those Infiniti bass-link and use that as a sub.
I have my factory radio in my audi with a 10' sas tube and its more the enough for me ( well soon will replace the stock tape deck unit with a more up to date one )
if you need Ideas PM me.