One problem related to underhood wiring has been reported for the 1999 Toyota Sienna. The most recently reported issues are listed below.
I was driving down the road when my van cut off and would not start back. I coasted into a driveway and smoke began coming from under my hood. I got out of my van to call my husband. By the time I had hung up the phone, there was black smoke pouring from out under the hood and it suddenly caught on fire. I immediately called 911, and the fire department came within 5 minutes. My van was a total loss. My van was fine when I left on this short trip, but then within 10 minutes, it was destroyed. We do not know what caused this fire.