hey guys i have a 96 blazer and the radio only works on the left front speaker
sometimes i get on the car and they all work but 80% of the time only front left speaker only
i thought it was the head unit so i got a pioneer and installed it all the wires r fine
and same thing is happening why isnt the front right and rear speakers working
is there anything between the headunit and the speakers that is doing that?

 

Well the speakers could be blow or there could be somthign wrong from the HU to the speakers. I cant imagine all the wires to the speakers have gone bad which would cause this so its probably somthing off the HU to the individual speaker wires. That or the speakers could be blown, so I would confirm the speakers are blown before you start ripping the harness apart.

 

My guess would be that either you have a loose connection or wires that are broken inside the insulation.

 

but am having the same problem with the original head unit and the new 1
is weird cause they all workbut most of the time is just the front left

 

If you're having the same problem with both head units, it's probably not the head unit, which is why aI suggested a problem with the wires or the connections.
Do you have an ohmeter to check continuity of the wires?

 

wouldn't even bother testing al speakers. would just run new wire to everything and start from there. sometimes, if you have a blown speaker, will creat a dead short and kill all the others. but my guess is that you have a combination of bad wiring and bad speakers.