Reverse lights keep blowing fuse...any ideas why?

I have a 2001 325ci I just acquired that have the OEM red/clear taillights with all new bulbs from the dealership but for some reason the reverse light fuse keeps blowing. The pigtail part of the harness was checked. No loose wires or burnt grounds. I even called the dealership and the 2001 325Ci's were not apart of the recall. The harnesses have never been cut or tapped into. All other parts of the taillights work perfect. I thought it was because I tried to use LED bulbs so I took them out and replaced with the oem stock bulbs and replaced the fuse and it worked fine for a couple days but then blew again. All bulbs good. Any ideas what could be causing this?