Thanks for all the advice. Keep it coming.
I don't think we'll sedate Stitches, seeing how he's in the car ALL the time. At least 2 or 3 times a day. He is used to traveling in the car and I"m positive the whining is not due to fear of the car or anything.
Because we had his behaviour in the car on the right track (ie pick a spot on the back seat or floor and stay) before Sasha died, and the whinning started right after, we're sure it started as him looking for her. It's been almost two months now. Sasha rarely went in the car with him, if ever. She used to love car rides, but in the last couple years as her arthritis got bad, she only went when we made her (ie the vet). Stitches only came to live with us in the last year, so I am fairly certian that the two have never been together in the car. So the behaviour doesn't make sense.
Talking to him, etc in the car just makes it worse. As for the Harness Idea, the car does not have locking belts in the back (only lap belts). We thought of that. We've also tried crating him, but that makes it worse. While hes now in one place the whining gets louder and he still tries to jump to see out the window.
ikes...lost about what to try next.