Rewire your speakers with a heavy gauge wire. (just incase you go to aftr-mkt speakers somewhere down the road)...
Then rewire the main power of the radio to include a new ground wire back to the negative post on the battery. (what happens, sometimes, is dirt and corrosion gets in the mounting area of the cab and make bad connections for the ground path. As the above thread stated re-do the ground for a good current flow).