1994 GMC C1500 low voltage

i replaced altenator and battery, still when i start truck cold volts say 14 after a fer minutes 13 when put in drive if i use radio heater or ony other acc. it lowers to about 12 and if i use any thing else it fluctuates between 13 and 12 but from time to time if hold any buttons it drops to 11 or below turn blinkers on same thing i have checked all ground wires from batt to body to starter and clean them still problem exist help me please