Creating a driver on a constraint with python? I have the following code and want to add a driver to the rest length of the stretch to constraint (bone constraint), but the console says that the property path is invalid. What am I doing wrong? 

armature = Name of my armature
str(GrpName)+"_bone" = Name of the armatures bone

BaseLenghtDrv = bpy.data.objects[armature].pose.bones[str(GrpName)+"_bone"].driver_add('pose.bones["'+str(GrpName)+'_bone"].constraints["Stretch To"].rest_length').driver


The code before the driver_add parentheses is definitely correct, I tested it with the rotation quaternion of the bone and it worked. But it doesn't work with the bone constraint.
Add the driver to the object, the data path is relative to that:

# We want Object.pose.bones[...].constraints[...].rest_length
# So use an object and the data_path 'pose.bones[...].constraints[...].rest_length'

BaseLenghtDrv = bpy.data.objects[armature].driver_add('pose.bones["'+str(GrpName)+'_bone"].constraints["Stretch To"].rest_length').driver