The truck is at Trancas St. and California (ave?). In the auto parts store parking lot on the SE corner. It's there from 2pm to 9pm according to the sign. I can verify that it was NOT there at 1pm when I first went looking for tacos.
We had the Tacos Al Pastor and all three of us loved them. They're terrific. Nice marinade, nice sauce, good meat, tender fresh corn tortillas...the Radishes and pickled Jalapeno wedges are a nice touch. The operators (owners?) are friendly people. The price is amazing ($1.25 / taco).
There are burritos and quesadillas on the menu. The fillings include Asada, Beef Cheeks (cabeza?), pollo and some others. I always go for Pastor.
I'm a stand-up eater. This is definitely my favorite food in Napa Valley.
Taylor's Refresher is fun, but not nearly as tasty. I handed my 7 yr old nephew an extra straw, he said something like "whoo-hoo, I got two straws." and, naturally, I replied: "one for each nostril" and before you could say "NO, DONT!" he had a snoot full of sprite, watery eyes and a severe, but short-lived sneeze.
Fillipi's Pizza is a decent place to take a truckload of kids (I've got 2, my brother has 4).