Good sound system for 07 silverado? What 4X6 speakers should I use?

I own a 2007 single cab silverado. I'm planing on using a Kenwood headunit, 2 10" Alpine SWR-1043D subwoofers with a MTX RFL1200D amp, a new body style GM reg cab sub box, a Boss CAP2B capitulator, and JBL 660GTI speakers with a JBL GTO1004 amp. The problem is that the speakers didn't include 4X6 speakers for rear. So which speakers, if any, should I get to match the JBLs?