I have this 18" full dome car that was due for LED upgrades, so I thought I'd show the upgrade in pictures. First, I removed most of the original wiring as they piped track power to the lights and I want the output of the LED lighting module to go to the lights. One of the requirements, besides all the benefits of LED lighting, was not to have pieces of the car hard wired together, that's a PITA when you're opening them up to add passengers and the like.
I started with the outer shell, the LED light strip is connected directly to the spring contacts, all the other wiring was removed. In order to make a neat installation of the lights, I retain the old light bar plastic and install the LED strip recessed in the light bar. This blocks the light from the sides and makes for a neat installation, and still allows the light to illuminate the entire passenger compartment. When you look at the dome from the side, you don't see the LED strip, just the smooth side of the light fixture. I cut away all the intermediate supporting structures and just leave the sides of the light bar intact The connections from the LED strip to the back of the car are hot-glued in place to route them to the spring contact block. I also put a few spots of glue along the strip as these have been known to sag over time, I guess the 3M adhesive isn't all it's cracked up to be. 
This is the main floor of the upper deck, and it has the lighting attached to the bottom. I also retained that light bar to make a neater installation. The metal conductors under the light bar was retained as it was a convenient way to bring power to the other end of the car. I installed a connector at the opposite end to allow connection to the bottom level floor and seating as that's where the lighting control module is located.
The lower level floor and seating, note the lighting module installed at the right. That's really the only mod to this beside the notch in the bulkhead to clear the added connector.
The chassis, (using the term loosely for a piece of plastic), was rewired to repurpose the spring contacts to carry the DC power to lighting, the track power was directly connected between the two trucks. Track power is also routed to the AC connector that plugs into the LED lighting control module. Note in the middle of the car there are two small PTC programmable fuses, these protect the wiring in the case of a derailment that might otherwise cook the wiring. They're rated at 255ma hold, more than enough for LED lighting. The two connectors at the right plug into the wiring module when the unit is assembled, they're dissimilar 2.5mm plugs to avoid the possibility of mistakes. The lighting module is equipped with matching male connector headers.
The bottom pieces all cabled up and setting together, ready to finish the assembly.
Now the part that took as much time than the whole rest of the job!
Turns out that these cars are some of the cars that MTH thoughtfully painted the whole truck and then put them together. The only thing is, they forgot that pain doesn't conduct electricity! All the trucks had to come apart to remove paint from around the sideframe mounts, and on the top and the bottom of the springs. Since these are sprung trucks, the springs actually carry the power from the wheels through the springs to the fixed part of the sideframe. Then, there's the screws that hold the sideframes, they were also screwed down on paint, and needed it removed to make contact. I wondered why the original lights were so flaky, but I figured I was replacing them anyway. When the job was done, I found out why the lights were flaky!
The joy of all this is I have a while bunch of these D&H cars to do, if they all have the same painted truck issue, it's going to be a time consuming affair!
The finished car on my York display stand, this will be my LED lighting demo car. Since this has two light strips, I have the module cranked up to supply about 35ma of current to light the LED's. Not bad to go from .45A of current to .035A, get better even and flicker-free lighting in the bargain!
