Michael...I did a FCC search for MVEC and only found permits for microwave transmitters! Their old 47 mhz system permit has expired, so can't be those. MANY years ago I was a dispatcher for them one summer...involved more knowing lines, sub-stations, loads, etc, than dispatching repair trucks, but was a blast for a summer job.
Back to the present...in the spirit of RR's beginning, you might start hunting for their radio use. There are several trunked systems in use down there with no TG info...they might be on there. Next time you see a repair truck, check the antenna and try to determine the freq, that would be a good start. Compile the info, double check it and then submit it to RR! I suspect starting with the Hidalgo Co. listings would be a good place to start, as the main office is in Mercedes. Post if you get interested in doing this and need help.