Here’s my rationale:
The problem is that warm, moist air is being pulled actoss the room and towards a large cold surface. This cools the air and causes the moisture to condense on the window and wall. What @Cupcakes is asking of a heater is to (a) reheat the air, (b) reabsorb the moisture on the wall and window and (c) create sufficient thermal convection current to overcome the existing downdraft (which is increased by the placement of the fan).
I just don’t think a little electric towel radiator is going to do that and the problem needs to be tackled at source. But if anything, putting a heater above the window might keep the air warm for enough time for the extract to do its job might be better.
Another option perhaps is to find some kind of hot air fan aimed up at the window, or even a demister like the electric circuit you have in a car windscreen? Idk if that exists but those might work better imo.
Edit: just try a bigger fan first ;)