There'* the engine sensor ground located on the rear horizontal bolt of the power steering bracket on the 00+'*. In the 96-99'* it'* the same hole, but a different length bolt because we don't get the ps bracket.
I went to one of the ground busses in the cabin. Passengerside and then soldered the wires to it. A little overkill, but I wanted to know the AFR was correct because my motor is worth a little solder to me. Plus I was anal retentive that day.
Being that both of you guys are 00+ and sharing the sensor (right?) I'd use one of the ground splice packs either inside or outside. They are all over your engine bay. ie ( by the ICM, on the trans by the selector, etc etc.
00 Regal GS: All kinds of cool stuff.