Your radio and fan may have shoted out or the wrong amps are being used. If there isn't enough amps to run everything. The battery might not have enough juice, or there are wires touching somewhere.
Depending on the car....but....for example on a 1990 Mercury Topaz, it is located just to the left of the battery (as you are looking down at it) and tucked up against the radiator area....just look...