My own experience
Being from the other side of the pond as you guys, I don't often join in the conversation, but for once, I have my own relevant experience to share.
Several years ago, I had a '96 Thunderbird that had a key fob. One day, having stopped in to get a bite to eat, I hit the button to unlock the doors on my car while I was next to a 2000s (not sure exact year) Chevy Impala. This made the Imp's alarm sound twice before popping open the trunk (a.k.a boot). The Imp's owner - sitting in the outside dining area - looked just as surprised as I was. Before then, I thought that it would be pretty close to impossible for a key fob from one manufacturer and one decade to have any effect on a car from another manufacturer and another decade.