Dick hits Mary on the back of the head so she forgets everything from the past six years. This causes her to go back the way she was before she met Dick. Before the aliens leave, they sing their anthem. The show ends with them being transported back to their own planet.
What are the words to the alien anthem?
"Across the void we come a warping,
Across the fields to stars we soar,
We pledge to land and something, something, spaceship,
And when at last our missions finished
In duty homeward shall we fly,
Our glory cannot be diminished,
Back to the heavens in our mighty spaceship!"Their motto is 'Proceed boldly and with stealth' followed by a salute rotating the palm of the hand around the top of the head and then down the forehead.
What happened to their car?
Presumably Mary kept it and everyone assumed the Solomans had run off to escape the massive credit card debts they amassed to pay for their goodbye party. In the final episode Sally tells Don that they are some sort of undercover agents which seems to satisfy him and explains their sudden departure.