If you have the mod resistor in parallel with the ECT, then when you unplug the ect, the harness still has a bridge resistor in it
[ the mod resistor].
This resistance value by itself is way to high for fan operation cut -in spec.

When the ecu senses an open [ which you do not have w/mod still in the uplugged harness] ,
it switches fan by default.

Remember, when you are putting the mod resistor in parallel with resistance of the ect, you are lowering the Total resistance
[at a given temp] of the ect and fooling the ecu..
In a parallel resistance circuit, the total resistance is always LOWER than the Smallest resistor in that circuit. So, if the cut-in spec of the ecu is , say 300 ohms, adding the mod of , say 1500 ohms, The total ect R. is now 250.
So , as you can see , the ecu will now detect an earlier cut -in temp by being fooled by the new temp/ohms set-up.