You don't need much documentation with you when crossing the Morrocan border; visas are not required.

You just need a valid passport and your car insurance details; green card and "permiso de circulación" (gray letter) - this is a letter saying you are allowed travel in certain parts of the country. If your spell in Morrocco leaves you wanting more, you should contact the immigration department and get the papers enabling you to stay longer then the initial three months. These papers verify that your passport has been seen by the correct border authorities. You'll get some kind of a stamp or number that you can show every time you pass through the border.