well you would need to use the editor coming with the game:

first of all F11 (Asset Browser)
-create a new "project" (left bottom of the dialog)
-search for the vehicle you want to modify and copy it into your new project

for example
em5/prefab/vehicle/ambulance_rtw .
-well when you found it in the Asset Browser you need to right click ->copy to asset package
-after you have done that you can save the rtw in the editor (else you cant)

Next step go into the prefab browser (menu bar->view->prefab browser) and place the model on the map
-also open the view->object proberties.
-then you can edit the model (the lights are linked to the base model

Screenshot 2021-03-21 19.21.57.pngScreenshot 2021-03-21 19.23.18.pngScreenshot 2021-03-21 19.24.03.png

-changing textures and implenting new models are also possible
-do you modify base game or a mod?

Textures
Skins einfügen (Anleitung)