[SWHouston]

On the Car Seat, here's a very simple inexpensive way to get that done...

Get a Cardboard Box, just tall enough to let her sit and look out.
Cover the top of the Box with an old Towel (for traction and softness).
Use your existing Harness on her.
Buy and inexpensive 6' Leash.

Run the Leash back behind the box, down between the seat bottom and back rest, and tie it to the framework (underneath) the seat.
Adjust that length where she can just sit/lie down comfortably.
You may place the Seat Belt around the Box, to keep it from sliding forward when Breaking. The Harness and Leash will keep her in place.

That should provide you with enough length/room, where she can move down on the seat, or over into your lap if you install this in the front passengers seat.

When not using this...
Leave the Leash tied there and just lay it down along side of the seat out of the way, and put the Cardboard Box in the Trunk if you want to for next time.