Hola Emma, I assume that you mean "la" in "la rieguen". "regarla" is a very colloquial Mexican expression that means: "to screw it up". I don't know your level of Spanish, but basically saying "voy a regarla" and "la voy a regar" means exactly the same. Thus "cuando la rieguen al volante" means that they will not turn ilegal immigrants to the immigration office if "they screw things up while driving the wheel". What it is not clear from your information alone is who's going to "regarla al volante"?

This comes from colloquial Mexican Spanish. When la/lo is usually in front of regar like that, it's like "to screw (it) up" so it translates like: El sheriff del condado de los Ángeles dice que no va a entregar a los indocumentados a la migra cuando la rieguen al volante. - The county sheriff of Los Angeles says that he is not going to turn in the undocumented immigrants to the Migra when (an implied "if"--it's subjunctive) they screw up at the wheel.

"la migra" is what they call the immigration officials in the USA