How do i fix my brake lights. They work fine until i turn my running lights on,

It's likely that the lamp socket has corroded causing weak grounding in the circuit, visit the auto parts store and purchase new lamp sockets. I personally performed this repair on my 2002 Express 2500 and chose to replace all of the sockets to the plug in design which is superior to the push and twist design. This is a common problem on these vans, if you drive behind one that hasn't performed the repair during night hours you will see that once the driver brakes, the lamps dim or switch off.