Sold my Sundown Audio SA-10's and looking at upgrading to a single 12" or 15" SSA Zcon. Not sure I can fit 4 cubes for the 15, but can probably fit in 3 cubes. What would be louder, the 12" be in 2.5 cubes or the 15" in 3 cubes? Either sub would be ran off a Sundown Audio saz2500d.

First go measure and see what usable dimensions you have available, that should give you a better idea. But from the users who own Zcons they leave bigger boxes so a 12" in 2.5-3 cubic feet may be your best bet. 

I had a 4 cubic foot box from an older sub but couldn't fit it in through the trunk opening. Maybe if I make the box more square than rectangular it might fit but I think I rather stick with the 12" being my size restrictions.  I can do 30 wide and 14 high and my depth can be a variable, but i think at around 25 inches deep or so I might run into trouble fitting it in and out. Was kinda hoping someone could figure up the dimensions for a 12" in 3 cubes at 32hz and I was gonna build a mock up to see if it will fit in and out.

Have you taken a look at any other subs? I've owned an Zcon, and they definately love larger enclosures / larger port area. So to answer your question more directly, I'd think you'd be better with an 12". 

 

Now, have you taken a look at the DSS Ethos sub? The 12" only needs 1.5 where the 15" only needs 2.5-3cuft. So, If you were to want an 15", that sounds like the best option, may also save you a couple bucks. 

Would you hsve the space if you built the box in the trunk?

Would you hsve the space if you built the box in the trunk?

Yes but not sure I can pull off a quality job lol