Ford F150 - volt swing re-visited

1990 f-150 i-6 4x4

have been searching for a few weeks now. have tested every ground i can find. all show o voltage under load. my final suspicion is the blower fan. it seems to illicit the biggest drop at the battery. anyone know off the top of their head where the ground for the blower motor is? it dropped about 6 inches of snow here last night and it was a little nerve racking sitting stuck in bumper to bumper traffic going home watching the alt needle swinging at 12. i had to keep the wipers going and blower on to see. thankfully my optima held its on.

i unno but i'm getting my volt meter around 12 volts when i run the heater, head lights and driving lights(100watt h3's), and even though, i can have the lights off and my volt meter will jump down in voltage and then go back up to normal 13 or 14 volt...