Major sound issue in 98' Ford Explorer

I have a serious sound issue on the entire left side of my 98' Ford Explorer...It has been this way ever since I got the car new and have taken it to Ford several times but they never can seem to find whats wrong with it.  I have recently installed 4 new pioneer interriors, thinking perhaps it was the factory speakers that was causing the problem.  After putting these in I notice the problem that much more.  The front left and rear left will be working, but be working very softly almost as if not working.  I turn the car on and everything is fine im driving along and I hit a small bump in the road and either the front left or the entire left side goes out.  To get them back up to speed I have to either hit another bump or turn the volume up above normal to get them back on.  I no longer want to push the limits of the speakers since they are new and I do not want to blow them.  I also plan on putting an amp and subs in the car, but do not feel like I should until I fix this problem.  HELP!!!!!

the 98 explorer has a factory amp in most models i think that maybe your problem you can A. order a new factory amp  that will be the most painless to install B. get  a 4 channel amp and install that 1974 ford pinto 4 15" punch Z power punch bd 1001 pioneer DEH-6700

or C get a head unit if you don't already have one and bypass the factory junk.2003 Chevy Avalanche,Eclipse CD7000,Morel Elate 5,Adire Extremis,Alpine PDX-4.150, 15" TC-3000, 2 Alpine PDX-1.1000, 470Amp HO Alt.