My all around mechanic told me it was my 1989 Westy's Alpine am/fm cassette stereo which is nearly 16 years old. So I bought a Sony am/fm cassette stereo on eBay and wow, the left side speakers still don't work!

I reinstalled the Alpine, only this time routing the wires for the left side to the right side speakers (and vice versa), and there's the left channel in the right speakers, but nothing in the left speakers. Actually the Alpine is a superior unit and should still work fine. I haven't used it that much.

OK, I get it, there's something wrong with the cabling to both the front and rear left side speakers. But where should I start looking and how to determine the cause?

Thanks!