Warning: The following is the result of a bored person. It should be by no means taked seriously.
Since we found out who Cartman's father is...the question now for him is who the mother is. Hypothetically it would be easy to find out who the mother is...just ask Cartman's ...uh...father (?). I mean only one person could have given birth and then handed him over, right?
I think the mother is obvious...if you look at body type and the love of snacky cakes...it has to be Sally Stuthers.