On 89's its at the junction of the secondary and primary pipes of the exhaust header on the driver's side of the engine. Looks like a giant spark plug.
and the original fault (stumble & occasional stall at idle) has stuck around...though nowhere near as bad as before. I cleaned the K&N with no notable improvement. I will try the EGR test today just to see.
The upshot of those plugs and/or the 02 sensor is an increase in MPG! Prior to that, I was getting around 17, and now getting around 20. That's like an extra 50 miles per tank if I did my figuring correctly, and that lets these repairs 'pay for themselves' in approx 20-25 tankfulls (about 4 months)