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".