Six fire crews went to the 1.5m sq ft (141,702 sq m) site in Bridgend – including firefighters from as far away as Cardiff and Pontypool – on Monday.
South Wales Fire and Rescue Service says the it appears to be in a compound on the site and not the main building.
The 40-year-old Ford plant is set to close in the autumn of this year with the loss of all 1,700 jobs.
Fire crews say they were called to the building at about 19:45 BST on Monday and South Wales Police said at 20:20 that the “fire is under control”.
Crews from Bridgend, Porthcawl, Kenfig Hill, Pencoed, New Inn and Whitchurch also attended.