Anyone have 2 15" in a 4 generation max? i was lookin to put two 15" in and wondering if you had to build a custom box or anything???

 

Generally 2 15" subs will require quite a large enclosure to sound good. At least 2.0 cu ft per woofer. There are some free air subs that would work well also as long as you do a diligent job of sealing off the trunk from the cabin. The gain in the total airspace will result in a subwoofer setup with very low bass extension, and a very smooth frequency response curve. Image Dynamics IDW 15's come to mind in particular as very good free-air / infinite baffle type woofers.