What is the teardrop-shaped appendage on the T-6 Texan?
It is the RDF antenna 'bullet' (or teardrop, actually) fairing. This image shows the (loop) antenna itself.



Source: mission4today.com

Initailly, RDF antennas were circular loops mounted above or below the fuselage. 



"Earhart-electra 10" by Originally uploaded by w:User:Logawi - Transferred from en.wikipedia, description page is/was here.. Licensed under Public Domain via Commons.

Later loop antenna designs were enclosed in an aerodynamic, teardrop-shaped fairing.



Source: airpages.ru