I'm not sure what small white tube you are referring to, but there is a white tube that attaches with a black rubber elbow to the top of the intake manifold and it goes back down to the transmission on the left side. Look for a nipple on the intake manifold that doesn't have any connector on it. Alternatively, if you idle the engine and spray starting fluid around certain areas where an open manifold leak exists you will notice a momentary increase in RPM. Sorry the harness didn't help.

__________________
1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles