Just curious, but isn't Benadryl for allergies? I'm curious as to what would make it effective in relieving car sickness. Of course, if it knocks your pooch out...I guess they can't really get nervous and throw up if they are sleeping.
I have found that short, frequent trips...to somewhere enjoyable will eventually ease a dog's nervousness in the car. Also, it's better to start when they are young...rather than when they are older.
Good luck!