Hands down the SDO rocker switch upgrade and the autosparks new harness (took care of so many electrical issues).
Rear terminal bars instead of bullet rat's nest connections, made adding trailing lights and other mods straightforward.
Running a dedicated ground loop apart from the engine grounds was a smart move, isolated the lighting system and made the LEDs reliable
TD5 gauge swap is much better on the 300TDI
Glow plug relay vs hold to start
Headlamp relay for obvious reasons
Ignition module with matching door cylinders (because who wants 3 different keys)
Rigid 'Chase light' which added a third brake light, reverse lamp, and an extra plate night I needed (did I mention the terminal bars where a awesome addition, made adding these ancillary lights a breeze).
Denali Sound Bomb horn.
Denali Sound bomb, Fit great on the radiator support brackets, really put some serious noise in front the the vehicle and due to it's modular design I was able to locate the unit in the engine bay.