Thanks for your replies. But the story (as I returned from Tunisia) goes like this:
The insurance is only necessary for on-road use, which would be the case had the quad not been transported on the trailer. So, insurance dismissed.
CPD. I wasn't talking about the CPD since Tunisia doesn't require one. Tunisia just stamps the vehicle (with its identification number, the easiest one being the registration plate) into the passport. ATA carnet was at hand to prove the ownership. ATA office in Slovenia doesn't know whether Tunisian Customs accept the ATA carnet for unregistered vehicles.
Actual procedure at Tunisian Customs:
All other vehicles were stamped in the passports with their registration numbers, the one in question with its chassis number and considered transported goods. No ATA carnet ever showed to the officers.
Upon exit, all the vehicles were cleared, including the one in question.
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