heater fan controls draining battery

I have an 88 Formula and when I turn on my heater the battery gauge drops 3 or more lines and the dash gauge lights dim. I have been stranded twice within one week with a dead battery. I had the battery tested and it is good but I replaced it with a new one just incase. I noticed the problem with my heater so I haven't run it much and its fine. I know its not the alternator. I took out the heater controls and unplugged the fan switch and turned it on and the battery guage didn't move then I plugged it in and dropped. It's not blowing out good like it should. I have a digital volt tester but I don't know how to use the thing. Thanks for the help.