Anyone ever built or used one of these? These what's, you ask?
A 9-volt battery, a diode and a cigarette lighter plug, wired up together. I made one
last night and used it today. Worked great. Plug it into the lighter socket, pull the
fuse for the radio (#3), then you can disconnect the main battery, do whatever work
needs doing, re-connect the battery, replace the fuse, remove the device from the
lighter socket and PRESTO! No need to enter the radio code, re-set the pre-sets or
re-set the clock.

I didn't invent it, it's been around for years and commercial devices are available
for about six times what this cost. My info came from an old VW Tech Tips sheet from
16 years ago.
But I didn't need it until now, when my recently acquired '88 Westy has the
Heildelberg radio, with anti-theft code.

Well, you can see a diagram and even get the RS part numbers here (click on code
saver):
    http://home.earthlink.net/~azsun99/jerryspage/index.html
Jerry