I need help with Devil Dog.
I am stuck.
It's his behaviour in the car. It gt better, a new dog appeared in the car, it became much much worse, resulting in more actual bleeding from me 
He used to be scared of other dogs, this drove him to be on guard all the time. When in fight, flight or freeze my terrier would pick fight every time
With the help of a behaviourist we managed to get this mainly under control. He's now what Jean Donaldson would call a Tarzan dog, he wants to play, he understands that other dogs are fun, he does not yet have all of the necessary social skills to interact safely with all other dogs - he still tries dry humping on occasion and sometimes attempts to greet nose to nose. Again this is improving, he can now run free with a pack of well socialised dogs who are slowly showing him the ropes and every day I see new improvements in his social behavior.
To get him to the place where he runs with the other dogs, we have to put him into a car with them. This did not go well to start with. He was still fearful of being 'trapped' with unknown dogs. He went into fight, flight or freeze. He fought, because I would not let him get to the other dogs, he took his frustrations out on me
He never caused any major damage, just minor cuts and bruising, mainly to my hands. We muzzled him. He got clicks and treats for behaving well. This helped him. Still the barking ensued, but this became more excitement than fear, because he now knew the other dogs were no threat and he was going on a really fun walk. The barking is better when he can see out of the window, as the biting stopped, the muzzle came off and he was allowed on to the seat, where he could watch out of the window.
Now we've added a new dog to the pack, an elderly male who howls a lot. Devil Dog has taken the howling to be some kind of personal insult. He bit me and drew blood again because I would not let him kill the new dog. Don't get me wrong, I know he did not want to cause me any more damage than he did. He still showed bite inhibition, which is good. He has some control over himself. If he'd gotten to the new dog I don't think he would have shown any restraint.
Outside of the car he was mainly fine with the new dog, he tried dry humping it, the new dog did not like this and a little scuffle broke out, but was quickly brought under control and Devil Dog did not bother him anymore. This new dog is not the kind of dog who will put up with Devil Dog's lack of manners.
It's just in the car I need help. How can I stop him barking and biting me? Would a DAP collar or spraying the car with DAP help? Clicker training improves the aggression (although never cures it totally) but does little for the barking. We often have new dogs. I don't want to have to go through this each and every time we introduce a new dog.