If you are pre-wired for the JBL
woofer, that would be your best bet and easiest install. The woofer shouldn't be hard to find used but if you are not pre-wired I believe the harness makes the price ridiculous.
Only other option that I know of is a line-level converter. You tap one of the speaker outputs and a little box converts the signal down from speaker level to line level with RCA. This could be used as the input for an aftermarket amp and sub. Some amps even have speaker level converters in them, so you could literally tap the speaker and run it straight to an amp.
This is all from memory .. I'm not a subject expert so maybe something else is available I don't know about