right rear brake lights upper and lower don't work on my 2005 chevy epress 3500 cargo van why

check the brake light fuses first and make sure they aren't blown.

I had this same issue with a 1999 tahoe I bought. I checked everything it possibly could have been. After some rooting a round under the dash I found a wire harness that had been stretched to the point. It broke. The harness had something to do with the brake lights cause it was part of the brake switch. Come to find out there were hundreds of people that had this same problem but couldn't figure it out. Hope this helps with your issue