The main campsites is your safest bet. Where you would easily spot the overland vehicles, and judge for yourself whether they have space/capacity for a hitcher.
There are many, but the ones that we found popular were:
Rabat (Salé beach)
Casablanca (the downtown camping, in a compound)
Marrakech (as mentioned above, just north of the city)
Dahkla (Just before the city limits. This might not be as popular anymore, since the convoy procedures are scrapped.)
If you hang around at these campings, you're 100% sure to get a lift if you're a decent guy/girl, and prove to be resourceful in some way (e.g. can cook, good mechanic, speak french/arabic)...
Jan
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