Window film over windows does wonders for reactivity somtimes, not sure if it'll help you.

Some resource guarding is learned, but generally it's a biological function that can only be managed. (It sucks, I feel bad for pups dealing with it, I have one - we've done loads of training though and my pup has a great life. The most important aspect is management.)

Not sure if this will help in the car, but for my reactivity training I did a lot of treat tosses as soon as the trigger came into view. (And treats go away when the trigger does.)

This way you're counter-conditioning that trigger equals treats, and over time it can really help. I have one pup who whines at me for treats as soon as they see a strange dog. Because they're a glutton they do it even for familiar dogs, like, "Hey! I saw this dog! I'm checking in! TREATS!" and it's pretty adorable so I do sometimes treat at that moment or use praise.

Thanks for rescuing. We need more people like you but I'm glad to know your home exists.