We came into Dakar by ship from Buenos Aires in mid-July with a 2006 Land-Rover Defender. We also had to obtain a "Passavant", the carnet was not stamped. Possessing and showing a carnet greatly facilitates obtaining a passavant though, as I was told by the customs broker from Grimaldi Shipping.
If you don't have one, or the car is older than 5 years, customs CAN impose an escort for the transit to the next border, i.e. you get a satellite positioning device fixed on the roof, and have to take a customs officer with you when you cross the country. And you can't circulate in Senegal except to transit. That procedure costs around 75000 FCFA. We've seen a bunch of cars with that set-up on our route to Mali (mostly Mercedes' destined for the Malian market, not tourist vehicles).
I'm not scaring anybody off here, you should just know about this and invest (time mostly) in negotiations for a passavant.
Hope this helps
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