I still need that foil.  A lister emailed me he might be able to find
one.  If you can, I'm still in the market, thanks!

Here's the rest of the story:

Van Cafe replaced my leaking master brake cylinder.  When I arrived
to pick up the Westy, they apologized: Bringing the van around for
me, they noticed the tach wasn't working.

There was an old, badly-done repair to the foil at the tach
plug.  They had disturbed that repair in removing the cluster for the
brake cylinder R&R.  (They offered to give me some consideration for
the trouble, but I declined.  It's not their fault the old repair was
so crappy.)

They hunted around for a serviceable foil, but no luck.

They showed me the problem and said they have a guy that can repair
the foils, but he's only around on Fridays.  I was hoping to be in
L.A. by today.

I'm pretty handy with a soldering iron, so I asked if I could camp in
a shady spot behind their shop and try to fix it myself.  I always
travel with my butane soldering iron, lots of electrical tools,
several kinds of wire, and so on.

Why not just leave it until I can find a foil?  A friend is taking my
Westy to Burning Man in ten days or so.  She hasn't driven a Vanagon
before, so I think it's important to have the tack working for her, if I can.

I spent four hours yesterday afternoon trying one thing after another
to tack the foil traces to the plug, without luck.  The foils just
kept breaking, or the mylar melted and the traces shorted.

I stayed with family overnight, and this morning returned to Van Cafe
to try again.  In the end, I was able to strip the mylar back,
freeing half an inch of each trace, and tack-solder some thin wire to
the traces.  I secured the wires to the tach case as best I could
with strips of gaffer tape.  It doesn't take much flexing to break
those traces.

Then I took the pins out of the connector, trimmed the remaining
mylar, scraped them up good so they'd take solder, and tack-soldered
the other ends of the wires to the pins.  Put the pins back into the
plug, plugged it in, then secured the wires with more gaffer tape.

We stuck the cluster in the dash, and it worked.  Amazing.  But it
was six hours of painstaking work.  Now that I've done it, it'd
probably take three hours, especially using much finer wire than I had with me.

But there's no way it'll work for long.  Gaffer tape isn't
structural.  As hard as I tried, I couldn't completely immobilize the
wires.  Anyway, vibration can easily break the traces, I think.

So I really need that foil.  But for now, it's working.