SWA does have some heavy maintenance done in Abbottsbord, BC, CA (YXX) at Cascade Aerospace.
http://www.cascadeaerospace.com/main.htm
I am unaware of any maintenance done in Mexico....and based on SWA's tremendous maintenance record, I doubt that they would work with a MRO or other supplier without a demonstrated excellent record.
[edited to add last paragraph]
[This message has been edited by BNAtraveller (edited 12-30-2002).]