I will try to hunt something up, but in the meantime try googling. Pope Benedict believes in the sovereignty of nations and in the law. Try googling Ratzinger also. The Catholic Church in the US, imo, has become a real problem to conservative Catholics. Benedict is a conservative. There is a rift in the Church which pretty much aligns itself with the international and national split in socialist/capitalist ideology. You might also try looking up Liberation Theology, another thing Pope Benedict has written extensively against.
I think it is a big jump to assume that Pope Benedict doesn’t support the US Bishops because he believes in Sovereignty of nations and law. I agree that US Catholics have become increasingly liberal, which probably isn’t to Pope B16’s liking.
I don’t know of any public statements one way or the other from the Vatican on immigration. Not that silence on US issues is unusual, but I would think that because there are some Bishops advocating the breaking of immigration laws, they would provide some input (at least privately. I am not convinced that the Vatican would disagree with the Bishops on the immigration issue. I am not sure that I understand your reference to Liberation Theology, and what it has to do with immigration.