14" wide 14" tall. 8" deep (bottom) 3.5" deep (top). T50 Torx (i think) to remove the two bolts in the sides of the seat. You should use a shallow mount sub with a grill protector.

Here is my amp under that same drivers side rear seat. Obviously I wasn't finished when I took this photo. But now the three kids car seats back there have it all locked in, or I take another photo.