
You have my sympathy as our young terrier barks in the car and it is soooooooooo distracting. I have now bought a crate for him which has at least solved the problem of him jumping up and down like a jack in the box with each bark

Our GR is an angel in the car so Cooper's vocal accompaniment to every journey came as quite a shock.
I have tried lots of different things one of which may work for you -
I tried covering the crate but that didn't make any difference at all.
I tried praising the odd quiet nano second but there wasn't enough time to get the word out before he started off again.
I had a bit of success with singing loudly to myself so he could hear me even though he couldn't see me but after a couple of miles he started to howl

:D
I tried putting the crate on the back seat so he could see me but he just wanted to get in the back of the car with our other dog.
I tried a seatbelt harness but again he just wanted to get in the cargo area.
I taught him to be quiet indoors on command - works well indoors but not in the car.
I gave him a rawhide bone in his crate - he is quiet for a fair while and barks intermittently - most successful thing to date :)
I take him out in the car but don't walk him.
His problem is pure excitement - I take the dogs in the car mainly to walk them in the woods so he knows he is going for a walk and just cant' contain himself. When I park at the woods I don't let him out of the car until he has stopped barking which used to be a wait of around 15 minutes but is now down to 3 minutes :) He also barks for the first 30 yards of the walk which takes forever as each time he barks I stop and don't move forward until he is quiet.
On the way home he is as quiet as a mouse and I don't hear a peep out of him which is why I am convinced it is pure excitement that makes him bark. I am hoping that he will get better as he gets older.
I clicker trained the quiet command that he responds to indoors and he got that really quickly but I am not sure whether to use that method in the car as, although I can click a quiet moment I can't give him a food reward because I am driving and there is only myself in the car. If any clicker users are able to help with that one I would be very grateful.
Hopefully one of the above methods will work with your dog as it is very distracting to have that constant barking and is hugely annoying.