my voltage is really low on my 91 GMC 1500, but i took the batt and alternator out (which are both months old) and had them tested and they are fine, i checked the ground cable and cleaned it, tried running it without the amp and alarm and still bad. when i use anything power (lights, blinkers, windows, a/c, htr, vent, wipers etc) it gets really low. not sure what to do here. any ideas?