I'm sure this has been discussed a million times, but
I was given a f90bt pioneer head unit.

Before i start ripping my car apart, where exactly is the factory sub? And If I remove the factory head unit is it true I will lose my sub? Is there an external amp for the sub only?

 

Pretty sure your sub is in the rear deck. And i think this link will provide you with the assistance you need.

https://acurazine.com/forums/audio-video-electronics-navigation-22/keeping-factory-sub-when-adding-aftermarket-headunit-99-03-tl-326253/

Pretty sure its similar between the tl and cl. Not 100% sure as I havent owed either.