You wouldn't think it would be worth posting about, but it is.
The truck is located on North Figueroa Blvd, just south of Avenue 49 in Highland Park. It's usually (not Tuesdays) parked on the east side of the street at Sycamore Grove Park.
They only serve tostadas and seafood cocktails. The "mixtos" tostada is a combo of shrimp, octopus, scallop, some sort of lobster meat and (i think) little bits of fish proper. It's mixed with a little onion and some avocado.
The food is really fresh. Apparently the owner and his wife pick out the seafood daily and nothing is kept overnight (not so sure about weekends, however).
In any event, it's good, cheap and fresh. One mixed tostada was enough for me when I went. The park is nice and you can have a good little picnic on the benches.
There are often two trucks there, serving similar items. MAKE SURE you go to the blue truck. The white one isn't bad, but it doesn't come close.