2001 BMW 325 lights out

Remove the connector at the rear tail lamps and inspect the connector for corrosion, especially the large brown wire which is the ground. That is where the issue is with this model and for the way you describe the problem. My method to repair this issue is to hard wire the ground wire to the ground bus trace on the tail lamp assemblies or replace the connector and renew the tail lamp assemblies depending on if it was warranty or customer pay.