The Motor Vehicle Disputes Tribunal deals with disputes. So you need to find out whether there is any actual dispute before commencing a claim. That would involve raising whatever issue you have with the motor vehicle trader who sold your vehicle to you.
It may be that the trader addresses your issue to your satisfaction. There would be no need to take matters any further if that were the case. If the motor vehicle does not respond within a reasonable time or refuses to address your issue to your satisfaction then you may have an appropriate case for the Motor Vehicle Disputes Tribunal.
It would be sensible to raise your issue with the motor vehicle trader in writing. You should keep a copy of any correspondence for yourself, as well as any replies received from the motor vehicle trader. That way you have a paper trail that may help you to establish a dispute. You could put that evidence to the Motor Vehicle Disputes Tribunal to prove a dispute and steps taken to try to resolve it.
Template letter raising issues with motor vehicle trader
You can download a template letter raising issues with motor vehicle trader here.