5 channel amp. channel 3 not working. fix?? or send it back?

Bought an alpine pdr v75. Channel 3 is not working. Its only producing a whining sound that seems to change in pitch with the engine. I hooked that speaker up to a different door and it worked fine with a known good channel and good wiring. I used a different RCA hoping that might do it but no luck. I put the wires from channel 2 into the channel 3 terminals and that speaker made the same noise. So this leads me to believe the amp has a bad channel. Is there a fix for that? What can i do about this. I bought it off Amazon so i can send it back if need be but im extremely bummed about it and will be leaving town for work soon and dont want to be without it haha