Initially, the serial number of the Standard Stratocasters made in Mexico was located on the front face of the headstock. It featured a prefix composed by two letters, “M”, which stands for “Mexico”, and another indicating the decade (“N” for the 1990s and “Z” for 2000s), followed by five or six numbers.
It's important to keep in mind that the serial number of some Mexican series was located on the back side of the headstock but followed the same serialization scheme of the Standard model. 
Obviously, there were some exceptions, like the California Series Stratocasters, whose serial number had the “AMXN” prefix.
Some Mexican signature guitars followed the same serialization scheme, while others sported a “MSN” or “MSZ” prefix if made respectively in the ‘90s or in the 2000s. Oddly enough, some Stratocasters made in the ‘90s or in the 2000s featured a strange “MSN6” or “MSN7” prefix even though they were not made in 1996 or 1997. These guitars should be dated by reading the other dates on body, on the neck, underneath the pickguard or thanks to the 1996, 2004 and 2006 anniversary tags placed on the back of the headstock or on the neck plate.
At the end of 2009, the serial number of the Standard model was moved to the back of the headstock, and in 2010, Fender decided to merge the serial number of the Signature Stratocasters made in Mexico with that of the other Mexican Stratocasters and moved it on the back of the headstock. Thus, since 2010, Fender Stratocaster made in Mexico featured a serial number with the simple “MX” prefix followed by the last two digits of the year (e.g. “MX12” for 2012) in bold, plus 5 or 6 digits indicating the guitar. However, in the first three months of 2010, an all numerals 8digit serial number was used, with the “10” prefix in normal font, followed by six digits.
This scheme, clear and simple, actually is not a reliable method to date a Stratocaster. In fact, at the beginning of a new year the old decals were not instantly discarded and continued to be used into the new year. Usually new decals were applied only after ca. 4, 5 or 6 months.
Some Mexican signature guitars followed the same serialization scheme, while others sported a “MSN” or “MSZ” prefix if made respectively in the ‘90s or in the 2000s. Oddly enough, some Stratocasters made in the ‘90s or in the 2000s featured a strange “MSN6” or “MSN7” prefix even though they were not made in 1996 or 1997. These guitars should be dated by reading the other dates on body, on the neck, underneath the pickguard or thanks to the 1996, 2004 and 2006 anniversary tags placed on the back of the headstock or on the neck plate.
At the end of 2009, the serial number of the Standard model was moved to the back of the headstock, and in 2010, Fender decided to merge the serial number of the Signature Stratocasters made in Mexico with that of the other Mexican Stratocasters and moved it on the back of the headstock. Thus, since 2010, Fender Stratocaster made in Mexico featured a serial number with the simple “MX” prefix followed by the last two digits of the year (e.g. “MX12” for 2012) in bold, plus 5 or 6 digits indicating the guitar. However, in the first three months of 2010, an all numerals 8digit serial number was used, with the “10” prefix in normal font, followed by six digits.
This scheme, clear and simple, actually is not a reliable method to date a Stratocaster. In fact, at the beginning of a new year the old decals were not instantly discarded and continued to be used into the new year. Usually new decals were applied only after ca. 4, 5 or 6 months.
Mexican Stratocasters serial numbers
1990

MN0 + 5 or 6 digits

1991

MN1 + 5 or 6 digits
A few MN0 + 5 or 6 digits 
1992

MN2 + 5 or 6 digits
A few MN1 + 5 or 6 digits 
1993

MN3 + 5 or 6 digits
A few MN2 + 5 or 6 digits 
1994

MN4 + 5 or 6 digits
A few MN3 + 5 or 6 digits 
1995

MN5 + 5 or 6 digits
A few MN4 + 5 or 6 digits 
1996

MN6 + 5 or 6 digits
A few MN5 + 5 or 6 digits 
1997

MN7 + 5 or 6 digits
A few MN6 + 5 or 6 digits 
1998

MN8 + 5 or 6 digits
A few MN7 + 5 or 6 digits 
1999

MN9 + 5 or 6 digits
A few MN8 + 5 or 6 digits 
2000

MZ0 + 5 or 6 digits
A few MN9 + 5 or 6 digits 
2001

MZ1 + 5 or 6 digits
A few MZ0 + 5 or 6 digits 
2002

MZ2 + 5 or 6 digits
A few MZ1 + 5 or 6 digits 
2003

MZ3 + 5 or 6 digits
A few MZ2 + 5 or 6 digits 
2004

MZ4 + 5 or 6 digits
A few MZ3 + 5 or 6 digits 
2005

MZ5 + 5 or 6 digits
A few MZ4 + 5 or 6 digits 
2006

MZ6 + 5 or 6 digits
A few MZ5 + 5 or 6 digits 
2007

MZ7 + 5 or 6 digits
A few MZ6 + 5 or 6 digits 
2008

MZ8 + 5 or 6 digits
A few MZ7 + 5 or 6 digits 
2009

MZ9 + 5 or 6 digits
A few MZ8 + 5 or 6 digits 
2010

MX10 + 6 digits
10 + 6 or 7 digits A few MZ9 + 5 or 6 digits 
2011

MX11 + 6 digits
A few MX10 + 6 digits 
2012

MX12 + 6 digits
A few MX11 + 6 digits 
2013

MX13 + 6 digits
A few MX12 + 6 digits 
2014

MX14 + 6 digits
A few MX13 + 6 digits 
...And so on