[shoubidouwah]

You have ID of who was in the car and when, and a 'report' button in the app for the next passenger. Easy enough, if something is reported you have the choice to wait for the next one while the trashed car goes to be cleaned or ride anyway at a discount if you solve it yourself - e.g: put the trash in a bag in the boot of the car. After N reports attributed to you, you get banned.

[mejutoco]

Uber can have reporting as much as this, or more. I can think of some: the driver owning the car incentivizes keeping it clean; bad reviews for dirty cars.

This problem is orthogonal to selfdriving.

[splwjs]

open car door

trash car (or don't)

report trashed car

ride for discount

[elaus]

You forgot the last step: Get banned because the video surveillance shows the trash being thrown in after you've unlocked the car.

[DiggyJohnson]

Yes. But this is obviously asocial enough to deter most people. Significantly better deterrent than self-checkout, at least.