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passenger side catalytic converter near the rear.  It would tell you if the
cat was plugged and generating excessive heat.  I had Aftermarket Cats put
on my car just the other day, and the light flashed constantly.  You can
either wire the two wires together, or I guess do something with the relay
to fix it.

For pulling codes checking relays, fuses, and etc, its all on the passenger
side, underneath the carpet.  You'll want to pull the fuse cover off which
is the side panel next to the door, and then pull up the carpet that is the
passenger side foot well.  There you can see the Diagnostics port, and also
the Relay that should be labeled "Catalyzer Warning Circuit" I think.

On your 90 you should only have 1 O2 sensor in the x-pipe forward of the
cats, and then the 92-94 4.2's had 2 per bank.  Not sure what engine is in
there or anything, but the fellow who mentioned that the Wideband AF gauge
could be hooked in that spot may have a point, but not sure.  Technically
the Gauge needs a wideband O2, not a narrowband like the ECU uses.  not sure
how the P.O would have worked it.  The gauge sensor also would have to be
forward the cats.