Where are all of the truck dealers?
This list should contain all dealers from both the base game as well as the Going East DLC:
Big dealers:
- DAF: Glasgow, Amsterdam
- IVECO: Frankfurt am Main, Wien
- MAN: München, Berlin
- MAJESTIC: Stuttgart, Genève, Warszava
- RENAULT: Paris, Praha, Budapest
- SCANIA: Milano, Manchester, Hannover
- VOLVO: London, Luxembourg
Small dealers:
- DAF: Cardiff, Bremen, Leipzig, Salzburg, Zürich
- IVECO: Torino, Grimsby, Hamburg, Brussel
- MAN: Edinburgh, Birmingham, Dortmund, Calais, Bern, Kraków
- MAJESTIC: Bratislava, Szczecin, Rotterdam, Plymouth, Newcastle-upon-Tyne
- RENAULT: Lyon, Düsseldorf, Felixstowe, Rostock
- SCANIA: Aberdeen, Lille, Gdańsk, Strasbourg, Dresden
- VOLVO: Verona, Nürnberg, Wrocław, Osnabrück
The accepted answer is significantly out of date - a number of new truck dealers have been added, both in updates to the base game as well as new DLCs, and the game no longer distinguishes between big and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 and Road to the Black Sea DLCs:
DAF: Amsterdam, Bergen, Bordeaux, Bremen, Cardiff, Catania, Glasgow, Iași, Leipzig, Санкт-Петербу́рг (Saint Petersburg), Salzburg, Tartu, Zürich
IVECO: Brussel, București (Bucharest), Constanța, Frankfurt am Main, Galați, Grimsby, Hamburg, Helsinki, Klaipėda, København (Copenhagen), Le Mans, Roma, Torino (Turin), Wien (Vienna)
MAN: Ajaccio, Berlin, Bern, Birmingham, Calais, Cluj-Napoca, Dortmund, Edinburgh, Kraków, Linköping, Marseilles, München (Munich), Rīga, Taranto
Mercedes-Benz: Bologna, Bratislava, Genève, İstanbul, Калининград (Kaliningrad), Kalmar, Nantes, Newcastle-upon-Tyne, Plymouth, Rotterdam, Sassari, Stuttgart, Szczecin, Warszawa
RENAULT: Bourges, Budapest, Düsseldorf, Felixstowe, Kaunas, Lyon, Oslo, Palermo, Paris, Praha (Prague), Rostock, Szeged, Со́фия (Sofia), Toulouse
SCANIA: Aberdeen, Brest, Cagliari, Dresden, Firenze, Gdańsk, Hannover, Lahti, Lille, Manchester, Milano, Pitești, Stockholm, Strasbourg, Tallinn
VOLVO: Brașov, Göteborg, Graz, Limoges, London, Luxembourg, Napoli, Nürnberg, Osnabrück, Пловдив (Plovdiv), Turku, Verona, Vilnius, Wrocław