First, you must go to the posted coordinates to find some information. Here you will find a sign with the number of miles to towns named Phelps in other states. The physical cache container is located at :
N 42 AA.BBB W 077 0C.DDD
AA = 410 - (distance to Phelps, Illinois) + (distance to Phelps, Michigan)
BBB = (distance to Phelps, Minnesota) + (distance to Phelps, Kentucky) - 1600
C = 208 + (distance to Phelps, Missouri) - (distance to Phelps, Wisconsin) - (distance to Phelps, Maryland)
DDD = (distance to Phelps, Texas) - (distance to Phelps, Georgia) - 134
The cache is located in a place that is closed from dusk until dawn. Please respect these times.
Update 6/1/2015: Changed the checker to certitude.
You can validate your puzzle solution with certitude.