Mexican insurance from Belize/Guetemala
 

When we drive from California to Mexico, weve planned to purchase some Mexican insurance at the border. Our trip is to continue thru Mexico then into Central America and back again. When entering Mexico from the south, are there companies selling insurance at these borders? Or is it best just to buy coverage for the whole time well be travelling, even if were not in Mexico the whole time? Or if you buy say 4 months worth of Mexican insurance, when you leave the country the coverage is put on hold, then is valid again when you enter?
thanks for any adivce.