[Occasionally6]

Most likely the ECU earths the switching side of the cooling fan relay. The way to check is to find the wire from the relay into the ECU and measure the voltage present with respect to a ground - all wires still connected.

You should see battery/alternator voltage until the fan is switched on by the ECU. With the fan on, the wire will be earthed through the ECU and so have zero voltage with respect to ground.

To switch the fan on manually, connect the wire between the ECU and fan relay to ground via your switch. Leave the wire connected to the ECU - use a splice. That will allow the fan to be switched on in parallel with the ECU switching i.e. either the ECU or your switch will activate the fan.

The current drawn by the relay is small, so virtually any switch can be used.