Audio Systems: 300zx Factory Bose System, 1990 nissan 300zx, car stereo repair

Question
I have a 1990 Nissan 300zx with the factory Bose 5 speaker system. I was having a volume control problem (crackling) so I sent the radio/casette unit out to be repaired. When I received it back and re-installed it, the only speaker that plays is in the center console...the other 4 speakers are dead but they were working before. I heard or read that the 300zx has a speaker relay that powers the 4 speaker amps on. Would you happen to know if this is true and if so, where in the 300zx it would be located?

Answer
relay is next to the radio

why did you send the radio to get repair ?  this is a good radio that don't have  problems
the problem is on the amplifiers, they fail all the time

most likely they need to be repair  
go to car stereo repair site to read more and find help
good luck