anyone know a shop in Guadalajara that works on Chev Dmax

Have you check the Duramax forum group?
Another problem is that those small repair shops sometimes are not always the best.
A well known shop here in the Phoenix area did work on my 6.0 diesel - I followed up with a visual inspection and found a severe problem. Took it back to them. Also noted other minor sloppy work from them that I took care of myself.
Sometimes those shops can create more problems in the course of paid repairs.