On the '99 Tradewinds there are typically two data port connections.
1. Loose on the passenger (left) side of the engine in the battery bay. (photo
2. Inside the driver's side panel under the transmission selector (difficult to see even when you know where to look) mounted to the wall. (photo
When you call Freightliner they will need the last six digits of your VIN. It should start with an "F".