Phoenix to San Diego
Assuming that you will come through Yuma, I would suggest a visit to Los Manjares de Pepe (about 2 miles west of 4th Ave on 8th St) or another local Mexican place.
Tacos Mi Rancho is a little hole in the wall around 2nd Street on 4th Ave - just a couple blocks off the freeway (last Yuma exit- 4th Ave). Not my absolute fav, but pretty good and easy on/off for the freeway.
Juanita's taco truck on 8th St, about a mile west of 4th Ave is good for fish tacos and Mexican coceteles.
Warning - some of the larger Mexican places in town (Chretin's or El Charro) serve rather bland old style Yuma Mex, which is not to most folks taste.
Search Yuma on this board and you will find more info. Here are some blog posts (more info than you need) about dining options here (note Pupuseria Cabanas is closed Thursdays):