Chris and Tim are right. Get a visa in Rabat, it takes 24 hours and it is without any problems. The visa is no longer given at the border and you will be turned back if you try - saw two people having to go back to Rabat because of it.
Carnet is worth having but with a little hassle and few euros you can get a temporary import permit.
There are many checkpoints both in the Western Sahara in Morocco and in Mauritania so it is worth having a photocopies of your passport with your profession, car details - VIN number and registration along with make and model. Having these forms with you will save you loads of time.
Good luck ....
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