History

1977 saw the return of the Diplomat as a full model line rather than as the name of a particular body style. It replaced the Coronet in Dodge's new downsized mid-size lineup. The new Diplomat was based on the Dodge Aspen, designated the M-body. While the Aspen had the F-body, the M-bodies were related to it: the wheelbase was often identical and doors and body panels often interchangeable. Hence, another M-body sedan, such as the 1989 Chrysler Fifth Avenue, had interchangeable doors with the 1976 Aspen. Like the Aspen, the Diplomat had coupe and station wagon variants.
The Diplomat was offered with a base 225 cu in (3.7 L) six-cylinder engine. In 318 cu in (5.2 L) V8 form (and an optional 360 cu in (5.9 L)) it, and its Plymouth Gran Fury twin, were widely favored as a police car in the US. After 1984, the only engine offered was the 318 cu in (5.2 L) V8.
Following the demise of the Dodge St. Regis R-body in 1981, the Diplomat remained, becoming the largest sedan in the Dodge lineup, despite being a mid-size. Dodge would not market another full-size car until the Monaco debuted as a 1990 model.
In 1982, the coupe was discontinued and Canada's Plymouth version of the Diplomat came south of the border. The Plymouth Caravelle was offered in the US as the Plymouth Gran Fury.
As the 1980s progressed, fewer private customers purchased the Diplomat (in part because of a lack of advertising and also because people favored more modern models), and the Diplomat (along with the Plymouth Gran Fury and Chrysler Fifth Avenue) was dropped in 1989. Despite fewer consumers, the Diplomat (and Gran Fury) had another market niche - as fleet vehicles for taxicab and law enforcement use (this was the last RWD Mopar (minus the Dodge Viper) until the 2005 Charger used as a police vehicle).
Diplomats built from mid-1988 until the end of production were among the first Chrysler-built products to have a driver's side airbag as standard equipment, some two model years before the remainder of Chrysler's lineup (They were also among the only cars at the time to offer a tilt steering column with an airbag).
The Diplomat was discontinued in 1989, with the Dodge Monaco replacing it as Dodge's top-of-the-line sedan for 1990.