Hi Tommy,
There is another way around that issue.. since I've been getting my windows tinted darkest legal tint, I find I never really need to "dim" my rear view mirror.
I'm assuming the auto dimming mirror would require some external power whereas the standard mirror would not, but if no spare wire available in the standard SR in that area, you could easily splice into one of the map light power wires I guess. Getting an auto dimming mirror from a wreckers would be a big saving over retail I imagine and assume rear view mirrors from many of the other higher range Hyundai models like Elantra SR and Sonata etc.. may be nearly identical and therefore interchangeable.
However, I still reckon a dark tinted rear window does much the same job permanently.