Mazda 6 Stereo Issue

[Superrat]

I'm new to the forum and could use some help. My wife drives an '09 Mazda 6 with the Non-Bose stereo. Only 1 out of 4 door speakers work with only a little sound coming out of the front door tweeters. I've replaced the speakers but it did not help. I replaced the factory head unit with a different one from a bone yard and still had the same problem, no sound out of 3 of the speakers. The only thing I can think of is that their is a separate factory amp somewhere but my research says only the Bose system has an amp. Any help would be great.

Thanks,

John