I want to unhook the wire that keeps the blade running when you back up.

It would be illegal to give you this advice PLUS the person who gave you the advice could be sued in the event of an injury.

Sorry.

That is a safety switch on many tractors. None of us would advise someone how to make a mower more dangerous. If it's a Sears tractor some have a setting on the ignition switch called ROS, reverse operating system. Turn the key to ROS and the mower will operate I reverse with the blade running. What tractor do you have, brand and model please