Car is a 2012 Mitsubishi Galant.
Went to drive it one day and the battery was dead. Totally dead. Car fob wouldn't work, lights didn't come on, and turning the key didn't even make clicking sounds. Left it attached to a charger for 12 hours, but no change. Was approximately a year old. Took it back to where I got it from and got new battery. Installed and drove to work, left it in the parking lot and car started fine when I got off work. Drove home, left it in driveway, and car was in same dead state when I went to go to work the next day, but the battery could be charged. For the next two weeks, I unhooked the positive terminal every time I left the car for more than five minutes to avoid the battery draining.
Tested it one weekend by leaving it overnight again, same thing, but again, battery could be charged, but only after detaching one of the leads from the car.
After several weeks, I forgot and didn't detach the battery lead, left it overnight and the new (less than a month old) battery was dead as a doornail and could not be charged no matter what.
In looking for ways to identify drain on battery, was doing some tests and realized that in 50 degree weather when the car engine had not been on for 12 hours plus, when I attached the battery, the cooling fan was coming on. Left it attached for 15 minutes, and the fan never turned off. Realized that every time I attached the battery to the car over the past several days, I had noticed that the fan was coming on, but hadn't been sure why.
Figuring now that the fan is running almost all the time when the car is off. What is the most likely cause of this, and how can I test it? I do not have the funds to pay for a mechanic unless it's something I literally can't solve myself. Suggestions?