Yeah sounds like the thermoswitch in the rad has failed. You can test the it with a pair of pliers in the connector contacts, if you can get to it (bumper off), but with your aux heater you may have to put the front panel in service position to reach the switch.
Funny this thread popped up again - I've just drawn the fan mounting panel in CAD to send off for laser cutting in aluminium next week
I don't like the new fan logic though as there is insufficient airflow while stationary for the AC to work properly. I'm probably going to put a second speed controller in for the new fan instead so both fans have three different speeds, and switch the lowest speed off over 30mph so the fans aren't running constantly.