Based on the version and the request for a "piece," there are two primary mods this usually refers to for the 1.46 update: 1. EKO Truck Parts (Backwall Piece)
You can often find this on the SCS Software Forum or dedicated mod sites like ETS2.lt. 2. "All Truck Tuning" / Tuning All Truck & Trailer Package AZ Г–SSZES teherautГі TUNGOLГЃSA 1.46
It unlocks parts across all brands, effectively allowing "the tuning of all trucks" as your title suggests. Based on the version and the request for