As can be seen above, both make use of a Crown. In Liechtenstein's case, as a Principality rather than a Kingdom, the Crown is a Fürstenhut or Princely Hat. Neither country shows evidence of a Crest in its history compared to what we have seen with other Monarchies.
Belgium is a relatively new country let alone a new Monarchy and was made up of areas of northern France and/or the southern part of the Provinces/United Kingdom of The Netherlands, depending on how far back in history you want to go. Research of the Royal House of Brabant and the Region of Brabant - the main forerunner of what is now the country of Belgium and the source of the gold Lion on black for the Belgian Coat of Arms - doesn't throw up any reliable version of a Crest further than a bunch of black and yellow feathers which is pretty much the fantasy of what is affectionately called a 'bucket shop' within heraldic circles.
Liechtenstein is also a relatively new country. The area of land between Switzerland and Austria was purchased by the Liechtenstein Princes to increase their influence and power within the Holy Roman Empire. The Liechtenstein Family originates from virtually the other end of Austria and has been a Princely House for as long as anyone can remember. They have therefore, presumably, never required a Crest further to the Princely Hat.
The final blog in the series may possibly provide a bit of a surprise...