I am in the same situation. I am trying to find some answers on the fourms. I have a 19 Laramie with the hk system and 12" screen.
I went to a few local audio shops nearby. Pretty much they all suggested an all in one powered subwoofer box under the rear seats. They were trying to sell me on the kicker 46hs 10", kicker 11hs8 which is the kicker 8", and a rockford fosgate all in one.
The installers brought one out to my car and it just slides under the rear seat. They're actually pretty small. They were saying they should be able to tap into my line inputs using a harness from under the passenger seat. (Didnt get the harness #). Aside from that it should just be running the power/ground. They didnt make it sound too complicated, but then again. I havent done it yet. But I'll try to keep you posted as I go. Cheers and good luck