The early 70s Airstreams have a cast tail light assembly that was assembled with its light cans before it was installed. When you take the lens off the turn/stop light, you see the bulb end (back end) of two pop rivets. If you drill these out, it is still difficult to get the can out. This is due to the tight fight and the fact that these cans will probably be surrounded by a fair amount of corrosion. You can carefully extract them with a clamp, but be careful not to pull the inner flange ring (this ring has the three tabs for the lens screws) from the can. Once you get them out and sand off the corrosion, they go back in pretty easy.
This can has a central rubber device that holds the fiber optic sensor. In the can in the photo the fiber optics has come loose, but if it were intact the assembly can be unplugged from the can. If you install a disc-type LED, all the light is emitted from the outward facing surface, so the fiber optics will no longer work.
The backup light can (the outside can) comes out much easier.
Once you've modified your cans to contain the LEDs, resistors, and diodes, they can be connected back in using bullet connectors or solder. The tail light casting has an aluminum box behind it, between the casting and the inner skin, which makes moving the wire harness difficult and also makes it hard to snake new wire over to the marker light casting--I needed to do this because I added another backup light inside that casting. Making the wires accessible required a 5/8" hole in the middle of the casting and in the shell skin. Then a long piece of hanger provided the means for snaking the wire.
If you use a "bare" LED in the tail light circuit, you need a way to dim it when it's powered by the running light circuit. You also need to isolate the running light circuit from the turn/stop circuit. The hold bulb did this automatically because it was really two bulbs (two filaments inside the bulb). If you don't isolate the two circuits with a diode, the stop light circuit will ground out the running light, and vice versa the running light circuit will stop the turn blinker from operating. Why? I have no clue.
The photo of the installed lights clearly shows the additional backup light shining out the bottom of the marker light casting.
The next step is to install a switch that will allow the backup lights to be used as illumination around the parked Airstream. This requires two diodes to isolate the Airstream battery from the backup circuit in the tow vehicle (you wouldn't want to charge the battery through the backup circuit) and to prevent the Airstream from illuminating the tow vehicle backup lights, if you're still hooked up.