what is the biggest speakers that will fit under the back seat of a 3500 chevy? i have 2) 10" subs now. does anyone make a good shallow 12" or is 10" my only option?

 

Just some thoughts ahead...

A single 10" speaker has a surface area of 78.5 ² inches. You have two of them- resulting in 157²inches. Are you thinking of replacing it with a single 12" with a surface of 113²inches?
Before you are going to replace them, do the replacement speakers have enough air volume to breath? Does the "Vas volume" fits? If not- it will be waisted money. Bigger speakers must play with an appropriate enclosure. Check, before you are going to buy and maybe get disappointed.

 

kicker makes a cvt pancake speaker , minimum airspace got 2 12s in my 2500 hd and it bangs underseat down fire box got them from car audio factory.com

 

that sounds perfect. thanks for the info