Carbon-fouled plugs do indeed mean you're running rich.

Generally the oxygen sensors on Hondas can go "bad" without the ECU throwing a code and cause the mixture to be richened up. I don't know if Toyotas do the same thing but it's a guess.

It could also be a bad airflow sensor. I know on the older Toyotas that used a MAF sometimes the spring inside the sensor can loose some of its er, "sproingyness" and the sensor goes out of calibration. I don't know if your Tercel still uses a MAF or if Toyota switched over to a MAP by then but it's worth looking in to.