Can anyone recommend a resource for monthly trade data for Mexico from before 2010?
I had a look for a few places. Looks like the WTO have something relevant. I haven't looked at the dataset but hopefully it is useful. This only has annual trade. The World Bank also has some data also covers annual data.

http://imts.wto.org/CDS/gp/tsselection.aspx

I think this one has your answer. It has monthly data. Only 1000 records at a time but up to 5000 if you sign up. You will probably have to be a bit clever with your queries and then merge a few spreadsheets as well. Clunky interface but the data looks vast.

http://comtrade.un.org/monthly/Main/Data.aspx