CURE!!!
My Rocky does that. Such a headach and he
ruined the blinds to my motorhome too.

Unless you have someone that can
do the driving while you sit in the back
with Lucy.
That way every time "before" she reacts in the
kill mode, you stick a treat by her mouth
and have her concentrate on you.
(get her mind away from that other dog).
Hopefully in time she will start thinking the dogs
outside aren't that bad and she gets rewarded
when she does not act up.

(I never did this in the car, but it worked
for me when I used this method while walking Rocky in the neighborhood by gates with barking dogs.)