BN2T-4S
 

On the Defender 4000 aircraft, (a turbine aircraft based on the Islander) located high on the tail fin there is a small 'tail plane'. There appears to be small ice boots on this, however the surface itself doesn't move. Anyone know what the function of this 'tail plane'is?

I found one photo wih the object you described at:
http://www.geocities.com/air_corps/c..._defender.html

looks like antennea to me, with icephobic coating (like the radome). Too many other photos of Defenders without the objects to believe it is an aerodynamic fix for longitudinal stability, or the like.