Thanks for the reply. I removed most of the glovebox area this afternoon and I think you would be right with the resistor idea. It looks like the blower is connected to the resistor through the earth wire and the last owner, instead of just replacing the resistor (which might be forgiveable if they had rang Subaru as they want $213 for it) they've put an earth cable in to make it run at full blast.
I guess it's not connected to the OBD port, in my naivety I thought they might share an earth terminal that might have come loose.
There is another small wire that looks to have been cut next to the blower. It looks like two thinner black wires, one of which is cut and joined to a thicker white wire that goes into the ECU maybe? I dunno what that box is next to the blower motor, but there's a shed load of wires going into it.