A while ago I was wiring a couple of sets of 3 white leds into observation cars, which seemed to be a cumbersome fiddley procedure. So I decided to whip up this little board just to make it easier. It's 31 x 55 mm overall size, with pads to solder in a rear led, and two alternate locations for side marker leds. Which ones to use will depend on what fits the particular car best. The board would mounted up inside the car roof, near the back end, using double sided foam adhesive tape, CA glue, or something similar.
The idea is to solder flat face white leds in with the leads full length, then use needle nose pliers to bend the leads as needed so that they shine directly on the red marker bezels when the car is assembled. The board includes a spot for a load resistor; I have found something in the range of 330-470 ohms seems to work nicely when connecting the board to an overhead strip of 3528 leds powered by a constant current regulator board such as that sold by grj through Hennings.
There is also a pad for a 1N4148 or similar GP diode, to rectify the track AC if no constant current board is used in the car. If using DC you would simply omit the diode and jumper the pads. You could use a standard 0.1" pitch 2 pin header and connector for wiring, but simply soldering 2 short lengths of #30 or so wiring would be an easy way to connect it to the led strip lighting.
To get a decent price point I went ahead and ordered 100 boards from an Asian board house, of which I only need about 15. So I can sell small lots of these to anyone else who may wish to use them. The price will be about 50 cents a piece, plus a nominal fee for shipping. This is a small price to pay for the convenience they should provide. (OSHPark's price is about 13 bucks for three!)
Here is the dead simple circuit diagram.
The 3D view above shows 5 leds installed, but normally you would use only one set of the side marker mount pads, whichever fits the car best.