First of all, close the game if it's launched.
Right-click the game in Steam and click Properties...:


There, open "Local Files" tab and press "Browse...":


You'll find yourself in the root game directory. Browse to Bus Driver Simulator_Data\StreamingAssets and find Stations.txt file:

Now your task is to find Stream URL of desired stations. You can do that by searching "<radio station name> stream url". This guide does not provide any stream links and does not fully cover finding them. After you found the links, open Stations.txt file. It should look like this:


Remove all contents of these file and insert stream links of each station, one per line:


Save the file and launch the game. After you exit the garage in Career mode or begin a Scenario in the corresponding mode, press V on your keyboard to show Radio. Press Backspace to release mouse cursor, click Play button on the radio, tune to your desired station and adjust the volume. Have a good trip!

Press Backspace to activate mouse cursor. After that, press the radio activation key (can be changed in Settings, V by default). The radio control panel will open:


On this panel:

• Volume knob is used to adjust volume;
  
• Tune knob is used to manually look for stations added to Stations.txt;
  
• "<<", "||" и ">>" buttons switches the radio to the previous station, pauses playback or switches to the next station correspondingly.