Weird aftermarket stereo problem

I was at Walmartone last year and saw a stereo system with speakers for $35 so I bought 3 of them . I installed one in my 93 Mazda B2600 and one in my Dart .
What the heck .. even if they are trashy sounding how could I go wrong !
Well the Mazda one worked fine for a year then stopped playing sound. It would intermittently come back to life for a few minutes then die again . It is getting power and all the display functions work but no sound .
So I figure the amp died and swapped in the new 3rd one out of the box…no sound but display and buttons still work ?
Each speaker has its own neg/positive wiring not a common ground. I soldered all my connections and did the wiring correctly….
What am I overlooking?