Nothing needs to be done ahead of time. The only paperwork you need for the vehicle is the registration papers (V5C in the UK, certificat d'immatriculation or carte gris in French). The D16TER can be completed at the border if you haven't done the form online. Your insurance might be checked.
If heading south (rather than north) you need a Visa to enter Mauritania, which has to be obtained in Rabat and that might be what you are getting confused with.
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"For sheer delight there is nothing like altitude; it gives one the thrill of adventure
and enlarges the world in which you live," Irving Mather (1892-1966)
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