power locks. '99 mercurcy sable

my car has power locks. but when i push the lock/unlock button, only the 2 back doors lock or unlock. i have to manually lock or unlock the 2 front doors. what could be causing this? and how can i fix it?? This could be a number of things as in a broken wire to the front doors, bad actuator(s), relay(s) and so on.
I would likely remove the door panels and test whether the actuator is receiving a signal and work back from there.
And yes believe it or not a computer called the GEM (generic electronic module) either monitors or controls the locks depending on the features this vehicle has; remote keyless entry for example. So you may need a scanner to diagnose this?
Dan.

Canadian "EH"