Nuke Afghanistan's poppy fields, and you solve a lot of the world's problems. Raimondo is right, Coulter and Nehlen are wrong. Mexico isn't the "source". It's the conduit.
From the New York Times in 2013, here:
Afghanistan is already the world’s largest producer of opium, and last year accounted for 75 percent of the world’s heroin supply. “The assumption is it will reach again to 90 percent this year,” said Jean-Luc Lemahieu, the United Nations’ top counternarcotics official here.