As explained by @phoog in a comment, this is called the Surinder Singh route. It stems from a decision by the EU court of justice and not from a directive or regulation so you won't find a text spelling out the exact conditions. In some cases, it might even be necessary to go to court to obtain such a residence permit.
The original case and many subsequent uses of this principle happened in the UK and a handful of other countries that have especially restrictive rules for family reunification. Consequently the UK government published extensive guidelines about this. Under their interpretation, you have to show that you moved the center of your life interests to another EU country (and did not merely go there to evade UK law), so anything that shows that can be useful to strengthen your case. I don't know how such cases are handled in Germany but in principle, the same logic should apply to any EU country.
Alternatively, you can always apply for a regular German family reunification visa (Ehegattennachzug), living in Belgium does not make that impossible. The downside is that there are additional fees and conditions compared to family reunification under EU law, which is why many people pursued the Surinder Singh route in the first place. In particular, as a Mexican citizen, you would have to show that you speak some German.
While it does not spell out what the conditions are and how to avail yourself of this possibility, the website of the federal office for migrations and refugees explicitly acknowledges all this:
Keinen Sprachnachweis vor der Einreise müssen Sie zudem erbringen, wenn
- Ihr Ehegatte Deutscher ist und er zuvor durch einen längeren Aufenthalt in einem anderen EU-Mitgliedstaat von seinem europäischen Freizügigkeitsrecht Gebrauch gemacht hat,
Note that it says “einen längeren Aufenthalt” (a “longer stay”) without naming a specific duration. Working in Belgium is enough for your wife to have “make use of [her] right to free movement” (“von seinem europäischen Freizügigkeitsrecht Gebrauch machen”) so you have got that part covered in any case.