I've never put a subwoofer in my 99 but I will say this. The solid wall between the trunk and the interior of the car precludes the installation of a trunk mounted sub. You'll bother people outside the car more than hear it inside the car.
This compact subwoofer from Kenwood gets great reviews and could possibly be mounted behind or under the rear seat.
https://www.walmart.com/ip/Kenwood-KSC- ... 3=&veh=sem
That's the way I would go.