Electrical problem with horn on 1989 Nissan Pintara 2.4litre twin spark.

The horn short circuits to on and starts operating on its own. This is only during Winter and particularly when the air is moist at night and very early morning.. like 4am!! I checked the connections to the horns and cleaned and used WD40 on them. This seemed OK but the problem returned later that night... Has anyone experienced this before? What might be the problem do you think? Any help would be gratefully accepted.. This old clunker belongs to my aging neighbour who is on a pension and very economically challenged so it is just not worth throwing money at this car and she cant afford to upgrade.. Thanks :-)
BTW there is no alarm system attached just in case you were wondering.