error code F16 is a directional switch error
the vcm is seeing multiple input signals from the directional switch
mitsubitshi sent out a service gram about this error code

remove and check the directional switch continuity between fwd, rev, and neutral and make sure all signals are clean and no cross signals between each mode selected.
If the directional switches show any abnormality then replace the switch.
If there is NO abnormality then check the wiring and repair as needed.
Now the service gram does say that if there a bad signal to replace the meter panel but i am assuming that if the directional switch and wiring both check ok then replace the meter panel.
Then it goes on to say if the meter panel does not repair the error then to replace the VCM.

The reason you are seeing that difference when the seat belt is buckled and unbuckled is because it is tied into the VCM and the signal result changes as you turn on and off the seat belt signal which effects how the unit interacts with the drive system.