I found a substitute marker light unit and installed one on the van. The screw holes of the lamp are spaced the same as the original but the overall size is smaller. And it's nominally 24 V but does light up fine on 12.

The dual voltage thing led me down a rat hole of making the lamp flash brighter with the turn signal. Not sure if it was worth the effort, the brightness difference is obviously not as great as it would be if it simply flashed on and off.

Pics etc on the blog, first post has the circuit bench tested (and also I try out a new led front turn signal lamp)
https://shufti.blog/2017/03/25/vanagon-trivial-little-things/, 
And the later post has the marker installed on van
https://shufti.blog/2017/03/26/vanagon-led-rear-side-marker-light-prototype/ 

Alistair