William,
Assuming Tom is right (he is a very bright guy!), this tube would run from the fuel tank to the front of the car because, as Tom said, it returns fuel vapor to the engine. I'm not sure of it's exact path because my car is too old to have one, but it probably runs under the driver's side floor. Look for the other fuel and brake lines. The kickdown switch is the switch under your gas pedal that forces the automatic transmission to down shift when you floor it. Looking at that area from under the car, you should be able to find the lines in question.
Rodd
Powell, Ohio, USA
1966 230SL, Euro, Auto, Leather, both tops
1994 E420