Reducing fear of the car
Leave the car boot open with blankets and treats inside so that your pup can get in and out when they choose to. A few times throughout the day, walk out, sprinkle some treats on the ground near the car or give them to your pup by hand, then just walk away. Also feed them their dinner close to the car. Repeat until your dog is happy to walk to the car with you.
Next, start walking out and sprinkling treats into the boot for your dog to reach their nose in to get (at this point put them close enough to the edge that they don’t need to jump in to get them). Repeat until they are comfortable retrieving the treats in this way. Gradually start to place the treats further and further into the boot, so that your dog starts having to jump in to get them. Then just let them jump out again whenever they choose. Once they are comfortable jumping into the boot, start giving them their breakfast/dinner in there.
Next, when they voluntarily jump in, you can start to shut the boot for 1 second, then immediately reopen it and let them out. Gradually increase the time the boot is shut. Once they are comfortable with being in the boot for 30 seconds, start also turning the engine on during this time. Then just open the boot and let them out again.
Finally, start taking them on very short journeys when they voluntarily jump in. These can start as being round the block to back home, then progress to going somewhere very close for a walk, then further away for a walk. All journeys must end in doing something your dog likes at this stage (e.g. going for a walk, visiting a friend).