A quantity of what is believed to be herbal cannabis with a street value of £100,000 was discovered in the car near Lockerbie.
Officers detained a 23-year-old man in connection with the find late on Friday night.
He remains in custody and Police Scotland say a report will be submitted to the Procurator Fiscal at Dumfries.
It is understood the man is expected to appear at Dumfries Sheriff Court on Monday.
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