Dean, diconnecting the wire from the thermostat will not affect defrost.
The wire controlling that comes from the outside unit straight to the
air handler(inside unit). The wire for that in the unit itself is Black
on a Trane, but the installers may or may not have hooked up a black
wire to it to run into the inside. They may have used a white, though
itis possible that they used any color they chose. The wire from the
thermostat to the air handler that turms the aux heat on should be
white. If you can figure out which set of wires go to the stat, and
which goes outside, you can disconnect the wire at the air handler if
that would be easier for you than disconnecting it at the thermostat.
Hope this helps Good luck Larry