2003 Chevy Express 1500

note the diagram, is that the fuse? Take wire connector loose for right rear lamp assembly, does the fuse still blow? You don't have to blow all your fuses, you can pull fuse, use both ends of test light at the two fuse terminals. When you step on the brake if the test light comes on, it's going to ground somewhere and it shouldn't. Check the electrical connector, see anything melted or burned? I'm thinking your brake lights work w/o key. If the test light doesn't come on, the problem is after the connector, either the lamp assembly or connector. you could replace dark green wire from front to rear, depending on the problem. just my opinion.

Thank you