If a 1996 Ford Taurus 3 liter has insufficient EGR flow where is it and how do you get to it if is concealed?

Carbon clogging in the upper intake manifold is the key! My 1998 Taurus 24V DOHC had an "EGR flow insufficient." (P0401). I checked the operation of the EGR valve (removed it and actuated it with a vacuum source), the solenoid (drove it with a battery and observed increased vacuum output), the DPFE sensor (drove the intake manifold-side port with a vacuum source, while powering it with 5VDC, the output went from 1V to nearly 5V with about 0 to 5 psig of vacuum). I confirmed flow through the EGR tube to the exhaust by running the engine briefly with the EGR valve removed. Manually applying about 7 psig of vacuum to the EGR valve reduced engine idle from 1,100 rpm to 900 rpm (that seemed weak to me, I expected the engine to struggle more, or even stall). I removed the intake manifold, then removed the throttle body. Once I peeled off the throttle body gasket, I could clearly see that the channel where the EGR gasses flow into the intake manifold just beyond the throttle body was almost entirely clogged with a rather hard carbon build-up. I was surprised that I was getting even any response from my actuation of the EGR valve, given how badly the EGR path was obstructed. It appears to me that the clogging really only occurs where the exhaust gas cools significantly - right there at the entry to the incoming air. The build-up of carbon was progressively less as I traced the path upstream to the EGR valve on the intake manifold. The passageway from the throttle body to the EGR valve had some carbon, but fairly minimal. As was previously suggested by another, the intake manifold is likely clogged if all the sensors and actuators in the EGR path are operating properly.