Coverage question – mechanic driving customer vehicle rear-ended our truck (commercial loss)

I own a small trucking company and I’m looking for help understanding insurance coverage on a recent loss. One of our drivers was rear-ended while working. The other driver was operating a customer’s vehicle from a mechanic shop at the time of the accident. The police report states the other driver was at fault and speeding. Unfortunately, the driver died at the scene. Our truck and trailer were damaged, and we’re losing income while the equipment is out of service. From an insurance standpoint, I’m trying to understand: Would primary coverage typically be the vehicle owner’s policy, the mechanic shop’s garage liability/garagekeepers policy, or both? How does permissive use usually apply in this scenario? Is loss of use / downtime commonly recoverable in claims like this? State is Pennsylvania Appreciate any insight from adjusters or others familiar with commercial auto claims.