There are no hard and specific rules regarding the usage of flags in the diplomatic community. There are a lot of accepted norms used by the diplomats.
The only relevant mention of flag application on vehicles by diplomats is Article 20 of the 1961 Vienna Convention on Diplomatic Relations. In that article, it is affirmed that diplomatic missions and their heads like Ambassadors or High Commissioners, have the right to use their national flags and emblem on their vehicles and premises of the diplomatic mission, as well as at the Head of Mission’s official residence.
That is all there is in that article 20 as far as the law is concerned. Fort he generally accepted practice, or call it unwritten conventions, there is nothing mendatory about it.
It only gives them the right to display their flags if they choose to do so.