Perhaps the current is reversed inside, or water got inside, or perhaps the magnet has a short inside. All a relay is is an electromagnet, and they can fail from weathering. Have you tried to substitute another relay to double check?
My 5.2L Build thread
-1989 Jeep Wrangler 2.5L, soon to be 5.2L
-1996 Jeep Grand Cherokar 5.2L, donor and rainy day driver
-2004 Kawasaki KLR650 sunny day driver
"A well regulated Militia, being necessary to the security of a free State, the RIGHT
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