Best option is to use the center armrest as the port and seal it off from the back of the trunk. You'll want to add a second battery (the factory battery is under the rear seat), the stock alt will be fine as long as you don't crank it all the time (the DTS has a digital voltmeter in the info console). Firing the subs/port towards the back of the trunk is the last thing you want in these cars IMO, it just made all kinds of rattle when I did that. I ended up putting the sub box against the tailgate firing towards the rear seat. I removed the layer of carpet behind the rear seats and removed the factory speakers on the rear deck to let bass through.
On and if that DTS is your own car then remember the active suspension on the DTS is hella expensive and that added weight will impact the lifespan of the struts. I don't have the DTS model so I got away with passive suspension but it was still a bit costly when I had to replace mine.