You didn't say what you have, but it sounds like you need a new mechanic. On my truck the back up lights are controlled by a switch mounted to the transmission. When a new switch is installed it is held in position with a factory installed clip which is removed after installation and ensures proper alignment of the switch. If the switch is removed and replaced later, there is a special service tool used to align the switch. If you have a new switch installed and it is not aligned correctly, the mechanic didn't know what he was doing and installed it incorrectly. Get yourself a copy of the factory service manual. It will give you the part number of the tool to align the switch. They are available on Ebay.