Stereo Wiring problem....

As many of you know i have a 1986 Isuzu P'UP Spacecab LS well i went to put in my bluetooth Radio and i wired it the same as the one i removed and well the volume is not working right..... Now this is kinda weird but its how this is wired... This truck if you hook the grounds up on the radio for the speakers the speakers will not work right they will not get loud and they will make popping sounds but if you cut the grounds at the back of the radio they play fine.. Well that was before i swapped the radios.. At the back of the radio the speaker wires there is no ground wires in the dash you would have to ground them all to the same ground.... My best guess is the speakers are grounded to the body somewhere.. Now what it is doing now with this radio is if you have the speaker control centered so sound goes to all 4 factory speakers the volume is low and won't get loud but if you adjust it to any one of the 4 speakers that one speaker will get louder and louder till you moved the adjuster to that speaker full the same with any of the speakers. needless to say i am confusedconfused

 

New electronic stereos can be difficult to figure out on older vehicles. My Cavalier has good sound out of the rear speakers, but the front sounds like their burned out. Highly distorted no matter what the volume is. I have a JVC stereo with CD player, came with the car.