[PowerPC603]

They still need to be informed that there is some cargo on that vehicle/trailer anyways, otherwise they see it as a free trailer found somewhere and don't know what to do with it.

Each cargo-type has a maximum of 5 drop-off locations linked to it.
Thus it would be perfectly ok to start a job immediately upon attaching it to your vehicle or getting into a vehicle with such a cargo onboard.
This would only work if there is no job started yet.
If the player already has a job, it could just detach the trailer or kick you out of the vehicle with a message to inform you about it.
By starting a job automatically upon entering the vehicle or attaching it to your vehicle (in case of a trailer), the player will know immediately what to do with it as his mission-infobar at the bottom of the screen will update with the required data that informs him about the cargo onboard and where to drop it off.

For these randomly spawned vehicles with cargo onboard, it doesn't even matter if that cargo is linked to your class or not. The cargo won't be any passenger-type, only cargo.
You only need to be able to drive that vehicle and you can deliver the cargo at it's destination and get paid.
Because the cargo would be random as well (a cargo-type that's transportable by that vehicle), you don't know what's inside until you find the vehicle/trailer and use it.
Also the quantity onboard could be randomized from only 1 unit till a fully loaded vehicle.
If you're in bad luck, the cargo could be 1 box of pencils inside an Elegant, earning you only a few dollars.
If you're luck is good, you might find a box-trailer filled with gold bars or diamonds, earning you a few 100K dollars.

But this system isn't implemented yet and will come after the modified job-system is working.