[SWHouston]

You appear to have gotten an animal with some bad habits...
and you're not going in the right direction as I see it.

First of all...
IF restraints are good for you in a vehicle, then they're good
for your dog !!!
Get a Harness, NOT a Collar, use a short leash to make a Tether
where you can restrain the dog in the back seat. Just make it long
enough to where the animal can stand and move some, but NOT be
thrown into the back of the front seat if there is an emergency stop.
And always have the dog restrained when the vehicle is moving !!!

Yorkies are creatures of habit, and once they have a habit, it's very
hard to retrain them.
DON'T GIVE IN, and start letting the animal loose in the car. Make the
back seat "it's" place to be, and play down the aggressive posture, by
NOT giving it a treat to stop, and use a firm corrective voice, with a
firm jerk on it's tether when it becomes aggressive.

This is just a start in retraining it, you got a ways to go yet.
but make that Dashboard totally OFF LIMITS always !