first of all, you need to have the complete link of the .mp3 player, for example, i want listen M2o (Italian radio), so i go to the player page and click on the left button of the mouse and after that i find the code of the player, i copy the ip and port of radio with .mp3 final.
IF you can't find it, search on google ip and port with .mp3 final.
Now, let's Work!
1 - Go in your Documents and open Euro truck simulator 2 / American truck simulator folder
2 - Open live_streams.sii with notebook and you will see some strings like this:
stream_data[214]: "http://bbcmedia.ic.llnwd.net/stream/bbcmedia_radio1_mf_p|BBC Radio 1|Top 40|EN|128|0"
3 - Copy a string and paste (or you can delete all and insert this once your favourite radio
4 - Set a number (in this example the number is 214, of the max id is 300, you set 301)
5 - Where you see http://bbcmedia.ic.llnwd.net/stream/bbcmedia_radio1_mf_pyou add the player link and after | where you see BBC Radio 1 you insert the name of the radio
6 - Save the file and if the work was done perfectly it should work!