Speaker question on 98 Suburban

My 98 seems to have eight speakers. In the front doors, each door has a tweeter and a midrange/woofer (6.5" my guess). Each rear door has one 6.5". Lastly, the rear compartment, in the roof panel, has two 4x10 speakers. Here's my question. Neither of the speakers in the rear doors work and this seems a little strange. Are they supposed to be hooked up and which fader setting controls them? Also, the 4x10's in the way back are junk - does anyone replace them w/6x9's? The 4x10 is a rare speaker now days and hard to find, let alone has a crappy sound.

Thanks