You have to pull back the trunk liner. I believe there are 3 10mm nuts that you have to remove to get the light out. Then you can remove the affected bulb and replace it.
I had this problem for quite awhile on my Bonneville. I finally got sick of replacing bulbs every other month. I was able to disassemble the plug and clean the contact surfaces. I then put some dielectric grease down into the plug before inserting the new bulb. Over 2 years later and I haven't had another issue, *knock on wood*.