Left side speakers weaker?

In my '99 Grand Prix GT, it seems that the speakers on the left hand side of the car are considerably weaker than the right ones (tested in both passenger and driver side seats). Generally I have to set the balance to 70/30 or 60/40 to get things to sound even. Is this normal, or should I invest in replacing the speakers? It's been driving me nuts.

all the speakers on the driverside are weaker than the ones on the right? Man, if this is the case, I'd wonder if the problem is the head unit. Is it stock or aftermarket? Just swap a speaker from the left side over to the right. If the left one is still weaker, then you've got a bad head unit or wiring.