Sorry to hear, @stockmax...Just curious even if it is in Mexico and spotted can anything be done at that point.
Couple of thoughts:
Obviously alert nearby departments.
Also Mexican authorities, however that works.
Unless it traveled inside another vehicle, I can't imagine that Border Patrol / Whatever doesn't have footage of your Cruiser and probably occupants crossing the border, if now in Mexico. Even without the proper plates, they should be able to bring it up very quickly unless they're more incompetent than I think. Whether they'll bother doing that for you is another matter. Alert them anyway (or have a PD do it, or both), in case the vehicle has yet to cross, or will cross again. Provide DETAILED photos (including any interiors shot through driver or passenger window) to them and all agencies. There aren't a lot of 80s out there. Fewer with stock roof racks. Zero others with whatever markings, rust, tweaked gutter or other damage yours may have in the same spots yours has it. If their cameras can see people 7 miles away, anything crossing at a checkpoint should be a sitting duck for instant AI ID.
If spotted in Mexico--and not saying I recommend this--some would be inclined to steal or buy it back. Might make for an interesting return-checkpoint conversation. Be damned sure to thoroughly search the vehicle before return. Maybe even call your home PD and let them know the score when on your way. You COULD call US border authorities. Probably just create problems if the vehicle is clean, but could save your a** if it's stuffed with something you didn't find...