I'm a U.S. citizen living and working in Mexico to help expand operations for a U.S. corporation I work for. I currently file and qualify for the Foreign Earned Income Exclusion under IRS rules. My question is if I have to declare any taxes in Mexico to SAT (Mexican IRS)?
You probably do. There may be some specific exclusions and exceptions under the tax treaty, but the general rule is that the country you do the work in (Mexico in this case) has the first right to tax your earnings.
I'm sure your employer can help you with reimbursement for a tax consultation with a local licensed tax adviser on this issue, and even if not - it is most definitely in your best interest to get such a consultation.