Finding out the production date of your mexican beetle is easy. All you need to do is to take a look at the rear right hand side of the rear apron (it's just under the hood) for the factory code which is stamped on it. It will be in the following form:-
ww = week number of the manufactured year (not the model year !!)
d = day number of the week (1 = Monday, 6 = Saturday)
ffff = factory code (I'm not sure of the significance of this one).
For example on my year 2000 mexican beetle, the factory stamp on my car is:-
0268057 = 02-6-8057 = Week 2, Saturday, 2000 = Saturday 15th January 2000
Note: Week 1 starts from the first full week of the year
The Model year starts from 1st August to 31st July (eg. a "model" year 2000 vw was made from 1st August 1999 to 31st July 2000).
There are other clues to back it up as well. Take a look at the wheels on your car too and you will find next to one of the wheel bolts the month and year stamp. My one reads 01 00 - January 2000.
The engine number will only give you a rough clue of when it was made.