I have seen people do this 2 ways.
The easiest route to me seems to be to use a 3 wire to 2 wire trailer light converter.
The other method I have seen is people make a new flasher/stop light circuit using 4 relays, but this method involves the front flashers as well as the indicator light.
Here is a decent link with the diagrams from the Samba:
http://www.thesamba.com/vw/forum/viewtopic.php?t=476562&highlight=tail+lightI am obsessed with relays, so I would probably go that route.
I created a new flasher circuit for my Ghia instead of buying the 60 dollar stock switch with 4 standard 12v auto relays.
I guess I never took a pic of that, but that link will give you the idea...