Brake Light Connected to Horn

[QuechuaGrl]

(I hope this doesn't seem too immature..) To make my story a bit shorter: I'm playing a prank on my friend. I found an idea where you connect the brake light switch in a car to the horn relay, causing the horn to sound whenever the brakes are applied. I'm not quite sure of what year my friend's Blazer is, however I'm sure that it is not very 'old'. If anyone could please tell me/inform me the process that is entailed, I would appreciate it! (unfortunately, I do not know much about cars) Is this illegal?-- who could I ask for help on this? Thank you!!