It would probably be something simple to put together.  My problem would
most likely be getting the connectors to mate to the EEC-IV plug.

With OBD-II now so popular, there isn't nearly as much of a need for vehicle
or brand-specific code scanners.  If you think you could find a market for
such a tool, I could probably put together a prototype fairly quickly.  I
don't really have the time available to make a one-off unit and the software
to communicate with the reader unless it was made well worth my while, and
something tells me you'd rather look into a blinking LED.  In all honesty,
hardware, including the enclosure for the reader circuitry, would probably
not top $15, and that's at one-off prices.  All that's required is a real
cheap 8-bit microcontroller, the wiring connections, and some support
components.

I also don't have the resources to allow it to do the fast read (later
EEC-IVs could spit out codes faster than your eye can see the LED blink,
which would be great for a PC ap).  That would be beneficial for PC
software.  It would also push up the cost slightly because of the better
microcontroller required.

IIRC, there is a constant monitoring mode, but it's only to monitor faults -
the EEC-IV is not designed to output operating conditions to the test plug.
You'd need a breakout box for that.  So, effectively, this piece of
hardware/software would be able to do just what your LED does, only present
it in a more readable fashion and cross reference the trouble codes to plain
text automatically.

JS