[my own comments marked by square brackets]
Informant: “One thing that I remember from my childhood is that my uncle would tell me that if you put an onion behind your ear, it would get water out of it. So if you were in the swimming pool and you had an earache from water being in your ear, a way to get that out was to put an onion behind your ear.”
[I’ve never heard of that. Have you ever heard anyone else say that?]
“I’m not sure. I think it’s mostly just from my family though that I’ve heard that. I don’t know why but they claim that it works.”
[Where is your uncle from?]
“From Los Angeles. Born and raised. Their parents too.”
Though my informant is Mexican by ethnicity, his family has been so far removed (physically and culturally) from Mexico that I don’t connect this folk remedy to Mexico. Instead I can speculate that it has more to do with the observable properties of an onion–strong fumes and ability to elicit tears from the eyes–which make it seem plausible to have a sensory effect elsewhere, even drawing water out from within the ear.