To access it:
1. Fold down the rear seat. The Lucerne's rear seat typically folds down in sections (60/40 split is common). You'll need to release the latches to fold down the appropriate section(s) to reveal the access panel. Consult your owner's manual for the precise location of the latches.
2. Locate the access panel. Once the seat is folded, you should see a panel covering the fuse box. It might be carpeted or have a plastic cover.
3. Remove the access panel. This usually involves simply lifting or pulling it upwards or outwards. Again, your owner's manual will show you exactly how.
Once the panel is removed, you'll have access to the fuse box.