radio hook up

[kglowacky]

just bought a factory radio for my 78 c10. got the pigtail with it. it is stereo. got 10 wires coming out of the back and red by itself hard wired. i confirmed by putting power and ground to the gray and black wires they are the the lamp power and they are on their own pigtail. the other eight i assume are for four different speakers, the hard wire red must be the power and it must self ground. can anyone confirm my thoughts on how this should be wired, thanks

Also is there a rear bracket that fastens the back of the radio to something or are the two front stem nuts the only thing holding it in place. if so can someone provide a picture of what it looks like.