A tug used to move ground carts collided with the left side of a twin-engine airplane while the airplane was taxiing on an active taxiway at night. The air traffic control (ATC) tower had cleared the airplane to taxi to the active runway via Delta taxiway. The airplane collided with the tug at the intersection of Delta taxiway and a service road. The tug was pulling five carts and was traveling from left to right through the intersection. The pilot reported that after the collision he saw a ground vehicle under the left engine and initially thought it was an abandoned vehicle because he did not see any lights on the vehicle. The pilot notified ATC concerning the collision, and then shut down the airplane. The tug driver reported that he stopped before entering the intersection and did not see any of the airplane's lights.