rear speakers

Hey guys, installed my 4 channel amp the other day. The speakers whine like no tomorrow when running, and whine when i touch the gas pedal when not running. I have tried a different deck, It only seems to come out from the rears, I've tried different rca cables. Ive reground all stereo...

so im looking for speakers for the rear door on my brothers extended cab but cant find the isze of them anywhere are they 6.75 or 6.5 inch?