Dogs develop motion sickness due to conflicting signals.
The inner ears sense movement, as does the body due to force felt during acceleration, turns, stops, and starts.
But, the inside of the car is not
moving.
When the balance-sensing system is thrown off this way, it can be very discombobulating, causing nausea. The nausea, in turn, causes the other symptoms including drooling, restless behavior, and whining.
Owners may make the mistake of holding their Yorkie on their lap to 'help'; but, this often exacerbates things, not to mention that is a flat-out dangerous.
Other steps may be taken such as offering more fresh air; however, as long as the car is still moving, it's hard to resolve motion sickness once it has begun. For this reason, your best course of action is to prevent it in the first place.