Need Sub recommendations plz..Just got a JL Audio 300/4 and pair of MB Quart speakers

Guys,

Old school setup! lol

I just got a hold of an old JL Audio 300/4 and a pair of old MB Quart QSD216 speakers that I plan on using for the front of my 04 TL (hopefully they fit).

I would like to add one sub to run in bridged mode for the other two channels of the 300/4.

What's the best way to do this? would a line converter work? Or would I need a sound processor?

Also, I have no idea how to set up the crossovers for the quarts. If anyone can help me with that, I would appreciate it.

It's been a while since I've messed around with car audio so bear with me.

Budget: I would like to spend maybe $300 or less for the sub. I know it's not a lot, but it's what it is..

Also, how to:

Tap into the factory amp for the leads to the front speaker and the rear sub.. Which wires are which? Which are the inputs to the factory amp? Does anyone have pics?

Thanks.