Where are all of the truck dealers?

The accepted answer is significantly out of date - a number of new truck dealers have been added, both in updates to the base game as well as new DLCs, and the game no longer distinguishes between big and small dealers. However, since my attempts at improving the existing answer have been rejected, I'm posting the up-to-date information as a separate answer. Not ideal, but there you go.

The list below contains all of the truck dealers from both the base game as well as the Going East, Scandinavia, Vive la France, Italia, Beyond the Baltic Sea, Road to the Black Sea and Iberia DLCs:

• DAF: Amsterdam, Bergen, Bordeaux, Bremen, Burgos, Cardiff, Catania, Glasgow, Iași, Leipzig, Санкт-Петербу́рг (Saint Petersburg), Salzburg, Sevilla, Tartu, Zürich

• IVECO: Almería, Brussel, București (Bucharest), Constanța, Frankfurt am Main, Galați, Gijón, Grimsby, Hamburg, Helsinki, Klaipėda, København (Copenhagen), Le Mans, Roma, Torino (Turin), Wien (Vienna), Zaragoza

• MAN: Ajaccio, Berlin, Bern, Birmingham, Calais, Cluj-Napoca, Córdoba, Dortmund, Edinburgh, Kraków, Linköping, Madrid, Marseilles, München (Munich), Porto, Rīga, Taranto

• Mercedes-Benz: Badajos, Bologna, Bratislava, Genève, İstanbul, Калининград (Kaliningrad), Kalmar, Nantes, Newcastle-upon-Tyne, Plymouth, Rotterdam, Sassari, Stuttgart, Szczecin, Valladolid, Warszawa

• RENAULT: Barcelona, Bourges, Budapest, Düsseldorf, Felixstowe, Kaunas, León, Lyon, Oslo, Palermo, Paris, Praha (Prague), Rostock, Szeged, Со́фия (Sofia), Toulouse

• SCANIA: Aberdeen, Bayonne, Brest, Cagliari, Dresden, Firenze, Gdańsk, Hannover, Lahti, Lille, Lisboa, Manchester, Milano, Pitești, Stockholm, Strasbourg, Tallinn

• VOLVO: Albacete, Brașov, Göteborg, Graz, Limoges, London, Luxembourg, Málaga, Napoli, Nürnberg, Osnabrück, Пловдив (Plovdiv), Turku, Verona, Vilnius, Wrocław