I see people discussing LED lighting, so I'll share this easy and (I think) cool circuit (schematic diagram attached).
I used this on my B300 interior cab lights but it should work on any old van.
I came up with it after thinking about how modern vehicles have interior lights that fade in (when you open the door) and then they fade out (after you shut the door) and how cool it would be to make my '73 do something similar. Also it takes away one of the unpleasant things about LEDs (in my opinion), which is the way they switch on and off so abruptly.
Anyway, it turns out it's dead easy
It's especially easy if you're doing it in place of the factory ceiling dome light because the main wires you need are already right there - a permanent 12v live and the connection to the switches in the door pillars. But you could use it anywhere. I was thinking you could even use a faster version on LED turn signals to make them behave more like filament bulbs. But I'll describe how I did it on the dome light circuit.
So basically it involves replacing the original dome light with a small relay (only needs to be a small one - my system only pulls like 40 mA with the lights fully on) and using the relay to feed the base of a transistor. The transistor switches the LEDs on and off but the resistors and capacitor in the circuit cause the transistor to switch on and off slowly, which gives the fade effect.
In more detail (for anyone interested) - when the relay flicks in, the easiest path for the power is down to the capacitor (through the 10K resistor) so most of the power goes that route and the capacitor starts charging up. As the capacitor becomes charged less power goes off down that route and it goes instead through the 39K resistor to the base of the transistor. So the transistor base sees a steadily increasing voltage and gently lights up the LEDs until it they are fully on (capacitor is fully charged).
When the relay flicks off, the voltage which has built up across the capacitor is no longer sustained and it leaks back, the only way it can go, through the transistor and down to ground. That holds the transistor on until the capacitor has discharged. There is a gradual drop of the voltage at the transistor base, so the LEDs fade off.
The values of the capacitor and the resistors in my schematic are the ones I found to give the effect I was looking for and should not need to differ too much for different LEDs. You only need to be careful with the value of the resistor at the top right where I used 480K ohms. That one will need matching to the LEDs. I used a string of little star shaped LEDs from battery operated christmas lights (I expect you know the type). They were running off three AA batteries so I knew they were designed for 4.5v and I worked from that.
Be sure to fit the resistor on the +ve side of the LEDs - they will work just the same either way but it's safer that way in case the LED wires get shorted against any part of the van. Or put a fuse in the main 12v supply wire.
I actually ran two LED strings along the trim panels above the doors. In the hole where the dome light used to be I fitted a switch so I can also operate the LEDs manually when the doors are shut.
Have fun !