The FCC has some empty allotments for Saltillo, notably 102.9 which is now 400 kHz between two operating FMs. 96.7 and 101.7 are available, but they are Class I ghost stations (Ramos Arizpe, Coah.) and so...yeah. 107.3 is in the Article 90 reserved band, so a public station could not be built there. 92.1 would be open, as would 88.1.
(It is worth noting that only one station in the 320-km border swath in Coah. was part of IFT-4, FID 94434 (90.5 Cuatro Ciénegas, Coah.). It should be a Class C per the allotment but it was awarded as a B1.)