charging issue or bad switch?

[68c10owner]

With the weather being much colder than it has been lately I`m using the heater daily now. I bought the truck in September and havent needed the heater until the last couple weeks. Anyway, I noticed that with each speed of the fan blower switch the voltage gauge drops a bit. Not sure if its a charging problem or if the blower switch is going bad. With the fan off it shows just below 13 volts but with the switch on high, which seems to be the only speed that works, it drops to just above 10 volts. lights are still bright and battery seems to still be charging fine but I cant afford any electrical issues right now. Spent $120 on a battery and didnt really have the money to do it.mine was dead from sitting while my tranny was being rebuilt and I couldnt get my charger from storage so it was easier to just buy a battery.