I would start with a large Chev dealer that sells trucks. Even if they cannot do the service they may be able to steer you onto someone who can help. I may be wrong but I think GM is now selling Dmax equipped trucks in Mexico. If so they will have the diagnostic equipment. Parts is another story. If they are not available in Mexico the parts would come from the USA to Mexico City and then to Guad. Some GM dealers in Mexico bypass GM. They have a Nextel phone and simply call a US GM dealer close to the border where they speak Spanish. The parts are carried across the border and fast shipped to Guad. If you are still under warranty make sure you save all the old parts and you can make a claim NOB. Remember this is Mexico and everything is possible!
Moisheh
