Edit : This method is kept for historical reasons. But do not use it anymore as Modding Tools have evolved since then (use Vehicle Manager instead).
Well,
here is a quick&dirty tuto to manually change brand names before Modding Tools will handle it easily (planned for next version).
1. Open File Browser.
2. Make a backup of your Database DB_xx.bnk file (where xx is your language code - FR = french, US = english , ...).
3. In your BNK_xx.bnk file contents, select the packed file: TDU_brands.xx (again, xx is your language code) then click on Edit button.
4. You'll see 2 new windows to appear. Let's go to the Database Editor window (the biggest one). Brands DB contents should be displayed.
5. In the list, check for brand names - names to modify are likely the ones between #81 and #140. I can't guarantee, yet.
6. Double click on the brand name to change, for example Wiesmann. In the entry editor, change ONLY the name, not the number. Ensure "keep original file size" is UNchecked. Finally click on OK to save.
7. After names are changed, go to the other window (Edit Tasks).
8. Click on settings button, then set the replace method to 'enlarge/reduce' if not already set. OK.
9. Check the task in the list then click on Apply button.
10. Don't forget to get a magic Map if you haven't installed it already.
Enjoy !