stereo problems

I just bought a '90 325i and it didnt come with the factory head unit. When i got it i wasnt hooked up right, so i went to go and redo it. The problem im having is the wire color codes at the speakers do not match the ones that exit through where the radio goes. Also there is only one set of wires for the rear speakers where the fronts have two sets. Does anyone know if this is correct, or should there be two sets of wires for the rear speakers?

the e30 stereo system has several quirks in it, i changed mine, or more exactly my wife's dad did, he helped me. he did it real quick like so i am not 100% sure of everything, but i will try my best. if i took my time to look at it i am sure i could have figured out, but i just let him do since he did it before, just so i can get it in there.

the e30 has a lower power amp. look in your trunk, on the drivers side by the power antenna motor. you will have to remove the trunk moulding. this is why the wires from the rear of the head unit are different then the wires to the drivers. if your head unit is over 40 watts a channel i would recommend totally disconnecting the amp. that is what we did. also you may have to look up more on this but he told me that some bimmers use something called a universal ground. with this system each driver only needs 1 positive wire, and i am not quite sure how the driver gets grounded. this is what we thought we may have troubles with in my e30. he said that if this were true the best thing to do would be to run another wire from the head unit to the driver that only had one wire. luckily in my e30 the drivers all had two, they just first went to amp. so we took the wire that went from the head unit to the amp and spliced them with the wires that went fromt he drivers to the amp.

good luck and let me know if i missed anything. sorry if it is unclear. .

:)