Hi @LATAM001
Unfortunately, TruckersMP is unsupported in running mods with the game, you can find rules §5.1: https://truckersmp.com/knowledge-base/article/704
If you want to get the Old Screamer Airhorn. Try to find some mods on the website, Steam workshop or downgrade the game version if you have that version profile backup; single play only
Hope this can help you.
All the best
PRC*LongmenInt'I.066