Hi Dave
First, using a 12 V bulb and 12 V power) you must locate where the problem lies (in which car).
To do this, you power the car fitted with a slider and check if it lights and if it transmits power to the next car (is there power on the Current Conducting Coupler (CCC)
If not, you need to open and check
The other cars are simply fitted with CCC and a ground return blade on one of the boggies
So the test is to feed the car with 12V between the ground return bogie and one of the CC. It must lit with both sides of the CCC making sure that the contact is good even when the CCC pocket is fully aside.
If not, you need to open and check

How to open the cars and fix some of the problems is shown on the video below


Checks on a faulty car:
- the CCC is correctly inserted into the NEM pocket while the extremity of the Y shaped contact with the Y piece (short coupling cinematic) goes also in the NEM pocket
- the cables are not properly installed RED cable to the slider (when there is one), RED cable to each end of the car on the flat brass plate (spade connector inside the toilets), BROWN or black cable coming from the ground blade
- the CCC is connected with the brass plate at each car's end, under the interior fitting. If Not the interior fitting must be removed, the brass plate should be removed by sliding it towards the car center VERY CAREFULLY as its hooks to hold it in place are VERY FRAGILE. Then access the Y shaped piece with its Y shaped contact blade. Caution, there is a return spring

See here the installation notice from Märklin https://static.maerklin....56b5256fb71560956594.pdf
Sorry to insist but these parts are very fragile and can break very easily
I hope this helps
Jean

This evening I opened the third and fourth cars. The fourth car, fartherest from the pick-up, was an easy fix, as connections had simply come apart. Within the flickering dining car, I found a brown ground wire bent and frayed right at the end of its connecting plug. I cut the bad section out and reattached it. It reduced the flickering somewhat, but not completely.
Am I right in assuming the issue is still the grounding in this car, as it still passes current to the car beyond it? Does anyone have any other insight or potential solutions?