I ran a few different sub stages in my last trailblazer. I just got another one monday, an 04. I recommend reinforcing the roof as well as getting an alt from like dc power say an xp270 I'm guessing you have a 4.2L since it's more common.
doing big 4 and a couple nice deep cycle batts. Northstars are on black friday sale at gp
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I'd say minimum 1 run 1/0awg from the front batt to the rear. Last TB I did 2 and probably will in this one. I run an ampere 5000.1 on sub duty and mids/highs I change up a bit but currently have a rockford T300-2 or something, birthsheeted 900rms 14v and is 1ohm stable.
If you're like me you'll want to quiet as much of the rattles a big system will do to a TB so I'd recommend buying whatever deadener you choose in bulk. I did 2 layers on my entire roof, than when it wasn't enough, I braced the entire roof by laminating it with 1/2" birch and rubber under broken loose supports including the one by front windsheild and rear hatch. You can get creative here. I used an old bodyboard fin since I already had it there in the ride.
Far as subs n amp, I'm a budget guy, I got my amp used off ebay prior had a sundown saz3500 I also got used. Subs I got an sp4 15 used locally, a DD 9518 used locally a sundown zv4 18 locally.
I liked the sundown though I've run fi for years. I still have a btl triple stack from years ago I recently reconed to a 10 a month or so ago. Really the box and install make a bigger difference than the gear when comparing similar equipment. In that respect, 2 12"s have more potential displacement and power handling than the same model in a single 15 and will work in a similar sized box. I'm supposed to be getting a pair of AA mayhem 12s soon locally (going to try to get rid of my btl first) and plan to do the roof of my new TB again as well as add more weather strip to the rear glass and the other stuff mentioned above + more I forgot already. I'd share pix but photobucket doesn't allow offsite hosting anymore or whatever tf it's called.
You'll also want a voltage meter at the dash somewhere, I ran mine off my amp inputs to know what my sub amp was seeing. Anything above 1,500rms I'd recommend an GOOD alt, not just some fleabay "big alt" or local car place, a proven, name brand big alt. Someone local perhaps if it's like singer, mechman, dc power ect.
As for the recommendation of X12s, if you can't find them for less on CL, try droppinhz they got some used ones on their sale section I noticed