Santa Cruz Taco Truck?
I don't actually think there's a legit taco truck that regularly parks in SC. The only one I know of is Ray's (?) that comes to UCSC campus at lunch time, but it's a bit more all-purpose (sandwiches, burritos, tacos) and I find them OK but not 40th birthday worthy.
On the other hand, one of the best trucks I've ever been to used to (as of a couple of years ago) park right at the corner of Del Monte and Hwy 1 in Marina. Awesome al pastor and lengua tacos. It seemed like others really liked the carne asada. Lunchtime on weekdays there were 2 or 3 trucks in this empty dirty patch on the mountain side of Hwy 1 (as opposed to the ocean side). Google Maps shows it's intersection of Lapis Rd and Del Monte Blvd.
Some people really like this truck in Castroville. The name escapes me but it was the first one as you go into town from Santa Cruz on the right. Intersection of Mead and 183, I believe (you can even see the truck on Google Maps satellite view). I thought it was good but not great, but others seem to disagree. Maybe worth checking out.
I don't know enough about Watsonville but there has got to be more than one decent one there. One day I'll have to do a taco crawl of Watsonville, brick and mortar as well as truck. There can't be more than, oh, a hundred places can there?