[Rainbow Six]

Mexico purchasing armoured vehicles from Brazil at some point sounds fairly plausible to me.

If we’re looking for valid reasons why the Mexicans would have Soviet armour as per City Of Angels I still think any of the following are possible

1. They were part of a shipment that were delivered to Cuba, either for use by Cuba’s own armed forces or the Soviets, and somehow or other they ended up getting shipped to Mexico. Maybe the Cubans were worried that the Americans might see them as some sort of threat (a possible invasion force?) and take pre-emptive action (i.e. nuke them) so struck a deal to give / sell them to the Mexicans.

2. A Soviet Op OMEGA. I think someone mentioned this up thread and it fits with my own opinion as to why Soviet freighters might be sent across the Pacific late in the War – Soviet High Command has decided that the Division Cuba troops could be of more use shoring things up back in the USSR so the freighters are sent to bring them home but there’s not enough room for all of their heavy gear so they agree to leave it for the Mexicans, unintentionally mirroring what the Americans did at Bremerhaven. Somewhere down the line maybe the whole thing gets called off (maybe the freighters all get sunk), but not before some armour has been handed over and possession, as they say, is nine tenths of the law.

2B. A less official OMEGA. Maybe a Soviet commander goes rogue, deserts his post in Texas and heads west with a number of a loyal troops (and their armoured vehicles). Their plan is to head for the west coast, commandeer a ship, and repatriate themselves to the USSR. Somewhere along the way they make friendly contact with a Mexican Army unit. The Soviets are ultimately successful (somehow) in taking over some sort of vessel but, as per 2 above, there isn't enough room for their vehicles so they leave them with their Mexican comrades in arms and sail off back to the Motherland.