Are you taking a car seat or renting a service that has a car seat?
If renting, be prepared for that seat (two seats, in our situation) to never show up with the car service, both on arrival and departure commutes (mainly on the departure commute. There will be a car seat in another vehicle at the airport on arrival, whether its someone's personal daily driver car seat, etc.). This will test the relationship with your wife/husband immediately. At this moment, you need to raise a shitstorm of anger and start yelling maniacally. In Tulum, there will not be another car for them to go borrow a seat from. It will take a lot of time to find one/have one delivered, and create a panic of potentially missing your flight.
Whatever happens, react however your spouse reacts. Staying calm will only make them angrier. You have to be a united front, regardless of how ridiculous it is.
Pro Tip: Take your own car seat, and verify that the seat that its attached too wasn't just welded in to the van 2 days ago. (i decided not to point that part out to my wife. I was already in enough shit bc I didn't do the whole united front thing.)