This info isn't broke down by monthly average but it might help.
See www.BocaChicaBeach.net/money.html for table or rates for every year since 1987 or <img src="http://www.bocachicabeach.net/images/money/peso_dollar.gif">.
Back then the official Central Bank rate probably was 14.02. It remained unchanged for a long while, although the street rate fluctuated quite a bit during the late 90's.
If you go to www.oanda.com they have a currency converter for various interbank through credit card rates and a very long historical database where you can even create and download the results, on as frequent as a day by day basis.
