I have also made a similar guide for American Truck Simulator, you can find it here:

This guide includes the truck dealers from the original map, Going East! DLC[blog.scssoft.com], Scandinavia DLC [blog.scssoft.com], the 3 new cities that were added with the 1.11 game update[blog.scssoft.com] and the 2 new cities that were added with the 1.19 game update[blog.scssoft.com].

There are 2 types of truck dealers in Euro Truck Simulator 2, a small dealer and a large dealer:

Large Dealer:


Small Dealer:


You can either visit the truck dealer by driving/quick travelling (if you have a garage in the city) to the city or you can buy the truck online, but this feature is locked until you have bought 5 trucks from the dealerships that you own at the same time.

The opening hours for the dealerships are Monday - Saturday 8:00 - 18:00 (8am - 6pm).

Note: If you are looking for a certain city, please use Ctrl+F and type in the name of the City.

Austria
Salzburg - DAF
Graz - Volvo
Wien - Iveco

Belgium
Brussel - Iveco

Czech Republic
Praha - Renault

Denmark
København - Iveco

France
Calais - MAN
Lille - Scania
Paris - Renault
Lyon - Renault
Strasbourg - Scania

Germany
München - MAN
Stuttgart - Mercedes-Benz
Nürnberg - Volvo
Frankfurt - Iveco
Dresden - Scania
Leipzig - DAF
Dortmund - MAN
Düsseldorf - Renault
Osnabrück - Volvo
Hannover - Scania
Bremen - DAF
Hamburg - Iveco
Rostock - Renault
Berlin - MAN

Hungary
Budapest - Renault
Szeged - Renault

Italy
Torino - Iveco
Milano - Scania
Verona - Volvo

Luxembourg
Luxembourg - Volvo

Norway
Oslo - Renault
Bergen - DAF

Poland
Kraków - MAN
Wroclaw - Volvo
Warszawa - Mercedes-Benz
Gdańsk - Scania
Szczecin - Mercedes-Benz

Slovakia
Bratislava - Mercedes-Benz

Sweden
Kalmar - Mercedes-Benz
Göteborg - Volvo
Linköping - MAN
Stockholm - Scania

Switzerland
Genéve - Mercedes-Benz
Bern - MAN
Zürich - DAF

The Netherlands
Amsterdam - DAF
Rotterdam - Mercedes-Benz

United Kingdom
Aberdeen - Scania
Glasgow - DAF
Edinburgh - MAN
Newcastle - Mercedes-Benz
Manchester - Scania
Grimsby - Iveco
Birmingham - MAN
Cardiff - DAF
Plymouth - Mercedes-Benz
London - Volvo
Felixstowe - Renault

If I missed a city or made a mistake somewhere please let me know in the comments below so that I can fix it!

Updates:

• 26/9/2014 - Added Switzerland.
  (Thanks pierrot2506.rohfritsch and evan!)
  
• 30/9/2014 - Added which garages are Small and which garages are Large.
  
• 7/10/2014 - Added ProMods V1.71 (WIP).
  
• 24/11/2014 - Removed ProMods truck dealership locations.
  
• 27/12/2014 - Updated the screenshots on the Small Truck Dealer & Large Truck Dealer in the 'Overview' section of the guide.
  
• 21/4/2015 - Updated the screenshots for Large & Small truck dealers and also updated the Guide Picture of the guide.
  
• 15/5/2015 - Added the dealerships from the Scandinavia DLC, removed the Small/Large dealership texts, replaced 'Majestic' with 'Mercedes-Benz' and added the opening hours to the Overview section of the guide.
  
• 26/7/2015 - Added the Renault dealership included with the 1.19 game update to Szeged.
  (Thanks whitecapsfan2010!)
  
• 4/8/2015 - Added the blog post links to Going East!, Scandinavia and 1.11 & 1.19 game updates to the 'Overview' section of the guide. And edited the 'Other Information' section of the guide.
  
• 3/2/2016 - Fixed a typo in Gdansk Scania dealership (Thanks Pelle Post!!) , fixed a typo in the Overview section , added the truck dealer opening/closing times , added the information that you need to own 5 trucks before the online dealerships unlock (Thanks v1per!) and added a note saying that I've also made a similar guide for ATS.
  
• 26/5/2016 - Changed the order of the Truck Dealers list to display the countries in ABC order instead of random order.