Well, it was time for a new O2 sensor.
Mine was acting sluggish and it turns out
that the heater was no good (open).

Since the OEM one is rather expensive
I figured I'd go with one of the "universal"
replacements with no plug (and some crimp
connectors).
I really don't like to use crimp connectors
on such critical (and low voltage) stuff, so
I shopped around a bit and realized that I can
get a 3-wire Bosch sensor for a Ford Mustang
GT (V8) for the same price as the universal sensor
(about $42). A 4-wire sensor for the Mustang LX
with 2.3L 4 cylinder is about $46. Cool....

I bought the 4-wire one for Mustang LX.
(Bosch 15716)

So, off to Pick'n'Pull I went, and snipped a 4-wire
O2 plug from a donor Ford and took it home and
grafted the plug onto my wiring harness and added
a signal ground wire from the plug to the block.

Now when I change my sensor next time I won't
have to buy an expensive one or fiddle with splicing
wires. I can just buy an inexpensive 4-wire sensor
for Ford 2.3L (very common) and plug it right into
my harness with no hassle.

P/N for the Bosch 4-wire sensor is 15716 and
it fits '91 Mustang LX with 2.3L engine, (as well as
many other Ford cars and trucks)

P/N for the 3-wire sensor is 13942 and fits
'91 Mustang GT (5.0L V8) and many others.

Make sure you get the right connector at the
wrecking yard.
All the Fords with 3-wire O2 use one style plug
and ones with 4-wire use a slightly different style.
They look very nearly the same, but are *not*
interchangeable.
The 3-wire plugs are black and the 4-wire are white.
(at least the ones I saw)

Get enough harness wire with the plug to extend
the VW harness about a foot to make up for the
fact that the O2 sensor itself has shorter wires
than the OEM VW sensor.
I soldered and heat shrunk all the connections.

Since my Westy has a Subaru EJ22 instead of
the 2.1 WBX, it was unnecessary to extend the
harness wires for my application.

Cheers,

Steve
EJ22 -> '86 Westy "Escape Pod"