In the last 6 months, my 2 neutered Jack Russells have had a few fights and recently had a very nasty fight in the back of the car, resulting in 1 being badly bitten by his eye and leg.
They are 3 & 4yrs old and the eldest is a very anxious, insecure yet domineering dog. He constantly tries to be 'top dog' e.g. always has to be the 1st to the front door, has to be the first to leave the house etc. He barks at every noise he hears and goes ballistic when the postman arrives...biting chunks out of my window frame! If the younger dog gets close to him while he's barking at postman/passersby etc, he will turn on him snappily. The fights have occurred when the younger dog has had enough & retaliated.
The elder dog is also a nightmare in the car...panting, whining, jumping around and gets absolutely hysterical if I leave them in the car, even for a few seconds!
The layout and situation of my home makes it impossible to stop the dogs seeing whats going on outside, so they bark A LOT! This and the fighting is becoming a major problem and I'm unsure how to deal with it. I'd appreciate any advice. Thanks

If the dogs can reach each other while in the car you would be best advised to adjust their harnesses so that they can not interact. (One at each end of the rear seat.)

The barking at postman etc can be dealt with by training the dog to be calm when post is delivered, with a stooge acting as postman you can make the dog sit and focus on you and a treat while the noisy post is delivered. This training takes time and repetition, probably best one dog at a time to start with.

Neutered or not JRT are working dogs. Many owners forget this fact as they only see them as small house dogs. They are a high energy breed that needs to be kept occupied otherwise they will find their own activities to amuse themselves. JRTs are hunters and diggers. A sand pit in the garden and training to find things would be suitable.