Convertible adding subs to stock hu

[onebadbird]

Here is my delimma, I have a black 99 Firebird convertible with every option avaliable from the factory minus the cd changer. I have the 8- speaker monsoon system, and i want to add subs using the stock head unit. I know I need a line output converter but where do i splice it in? By the headunit? before or after monsoon amp? where? Also i read that the monsoon amp was by the spair tire in the rear area but i opened that puppy up and i couldnt seem to find it? Please some one help! Could someone recommend a specific output converter or explain how they hooked up subs to their monsoon system without rewiring the whole thing??? ---thanks