Actually, there is a difference in digifant and digijet systems in this
regard.   Digijets ground through the exhaust system, as David says.
Digifants ground through pin 19 of the ECU, through a brown wire that emerges
from the same bundle as the distributor wiring.  This system is insulated
from the body of the sensor - you cannot replace the pin 19 ground by
grounding the sensor body.   If pin 19 can't find a good ground, the oxygen
sensor signal will vary wildly and the van will run terribly.  Many a
mechanic has blindly replaced AFM's and other sensors wondering why he can't
get the van to run right when the only problem was a loose wire on the left
head.  If you have a digitool, you can disconnect the pin 19 ground and watch
the oxygen sensor signal go haywire.

Bill
St. Louis, MO
www.geocities.com/wdummitt