Question: The Ford Explorer and the Mercury Mountaineer are very similar vehicles. What year did the Ford Explorer start being produced?

Context paragraphs:
Firestone and Ford tire controversy: The Firestone and Ford tire controversy was a period of unusually high failures of P235/75R15 ATX, ATX II, and Wilderness AT tires installed on the Ford Explorer and other related vehicles.

Ford Explorer Fuel Cell Prototype: Ford Explorer Fuel Cell Prototype is a hydrogen car based on the standard Ford Explorer.  The car was introduced at the 2007 Greater Los Angeles Auto Show, which was held in November 2006.

Lincoln Aviator: The Lincoln Aviator is a mid-size luxury sport-utility vehicle (SUV) that was sold by the Lincoln division of Ford Motor Company from model years 2003 to 2005 in North America.  A rebadged variant of the Mercury Mountaineer and Ford Explorer four-door SUVs, the Aviator was the shortest Lincoln vehicle ever sold (at its debut).

Ford Explorer Sport Trac: The Ford Explorer Sport Trac, sometimes referred to as simply the Ford Sport Trac, is a mid-size pickup truck sold mostly in North America produced by Ford from 2000 through 2010.  The Sport Trac was based on the Ford Explorer SUV - not based on the compact Ford Ranger (North America) - because it was introduced when the Explorer and Ranger platforms diverged (Explorer's chassis was a heavy duty variant of the Ranger's).  The Sport Trac fell between the Ranger and Ford F-Series in Ford's truck lineup, based on capability and price.  It competed with other mid-size trucks and SUVs, such as the mid-size Honda Ridgeline and even the larger full-size Chevrolet Avalanche.  During its first year on sale in the U.S., there was a waiting list of over 3 months.  The Sport Trac was a Class 2 vehicle with a GVWR of 6250 lbs.  and a maximum GCWR of 12,000 lbs.  This made it one of the more and the then most capable mid-size pickups, having a greater GVWR and GCWR than the Dodge Dakota, GM's GMC Canyon/Chevrolet Colorado pickups and Toyota Tacoma.

Ford 6R transmission: The 6R is a six-speed automatic transmission for longitudinal engine placement in rear-wheel drive vehicles.  It is based on the ZF 6HP26 transmission and is built under license by the Ford Motor Company at its Livonia Transmission plant in Livonia, Michigan.  The 6R debuted in 2005 in the 2006 model year Ford Explorer and Mercury Mountaineer.

Chicago Assembly: Chicago Assembly (frequently Torrence Avenue Assembly) is Ford Motor Company's oldest continually-operated automobile manufacturing plant.  It is located at E. 130th Street and Torrence Avenue in the Hegewisch community area of Chicago, Illinois.  Chicago Assembly currently builds the Ford Taurus and the Ford Explorer, both of which share the same platform.

Saleen XP8: The Saleen XP8 is a performance sport utility vehicle based on the Ford Explorer created by Saleen during the years 1998-2001.  There were three prototype models, based on the 1997 Explorer XLT, which when viewed closely, has distinct characteristics apart from the production models (rear hatch design, taillights, and integrated license plate into the rear bumper).  The Saleen XP8 comes in two wheel drive, or all wheel drive four-door configurations, packing either a 5.0-liter V-8, or a Saleen-developed supercharged 5.0-liter/286-horsepower V-8.

Jurassic Park (arcade game): Jurassic Park is a rail shooter arcade game developed and released by Sega in 1994.  It is based on the 1993 film of the same name.  The game cabinet resembles the rear of the Ford Explorer tour vehicles used in the film.  The player, equipped with a joystick, must shoot dinosaurs that appear on-screen throughout the game.

Mercury Mountaineer: The Mercury Mountaineer was a mid-size luxury sport utility vehicle (SUV) that was sold by Mercury from 1997 until 2010.  Sharing many of its features with the Ford Explorer, the vehicles were virtually identical in terms of hardware.  Externally, they were styled somewhat differently, and the Mountaineer was positioned with a more upscale interior, with the Mountaineer's MSRP coming in at $1,000–$6,000 more than the Explorer.  It was last redesigned for the 2006 model year with a new frame, looking very similar to its previous model.

Ford Explorer: The Ford Explorer is a full-size sport utility vehicle produced by the American manufacturer Ford Motor Company since 1990, based since 2010 on a crossover platform.  The Ford Explorer became one of the most popular sport utility vehicles on the road.  The model years 1991 through 2010 were traditional body-on-frame, mid-size SUVs. For the 2011 model year, Ford moved the Explorer to a more modern unibody, full-size crossover SUV platform, the same Volvo-derived platform the Ford Flex and Ford Taurus use although with significantly greater ground clearance.  It is slotted between the traditional body-on-frame, full-size Ford Expedition and the mid-size CUV (Crossover Utility Vehicle) Ford Edge.  Although outwardly similar, the fifth generation Explorer, Ford Edge and Ford Escape do not share platforms.  The fifth generation Explorer does, however, share platforms with the Ford Flex and Lincoln MKT.

Supporting paragraphs: Mercury Mountaineer Ford Explorer
Answer: 1990