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In order to find the source of the direct short you will have to do a process of elimination. First unhook the low voltage wires going to the outdoor unit where they hook in to the outdoor unit. Make sure they are not touching each other or anything else and turn the system to cooling like you normally would. If the fuse doesn't blow the short is in the outdoor unit either the contacts or the defrost control board. If it blows again then you move on to the thermostat.
hook the outdoor unit wires back up and then go to the thermostat and unhook the wires from the stat. Twist the red, yellow, orange, and green wires together and the cooling should come on. If the fuse doesn't blow the thermostat is the problem if it does blow then the problem is the furnace control board or the wires themselves going from the outdoor unit to the furnace and thermostat are bare and shorting against each other.