DIFFiculties:
The only way that the light can come on is if the Differential Lock is engaged. The
switch to the light is located on the end of the Differential Lock shaft (LHS of
Differential). It is a simple switch with a roller ball end which drops into a groove
in the shaft when the Differential Lock is engaged (shaft pushed in).
There is a good chance the problem is at the pneumatic actuator (RHS of Differential)
which pushes and pulls the Differential Lock shaft. If someone mixed up the two lines
from the dash knob, then the actuator would work backwards.

> It could be that the lock is on when the knob is pushed in, and you actually
> disengage the lock when pulling it out. This can happen if the vacuum lines are
> swapped from its places to the opposite; the function would be opposite too.