New Trailer wire harness = Brake Light on .. Help.

Just a the pick up wired for 8 wire harness to tow the camping trailer. About 15 mins from the house the brake warning light came on. It stayed on for the rest of the trip home. After about 20 mins
I headed off to my parents place, at first the warning light was OFF, after about 10 mins it came back on. <-- This all happened with the trailer hitched to the truck.
I dropped the trailer off at my folks place and on the drive home the warning light never came on once.
Any ideas?? wire from the brake controller? or else where?
Thanks Snow....
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Forgot to add..
the truck is a 92 GMC 1/2 ton.
The trailer brakes and controler are both electric.
I have driven the truck for 3 days since the dash light kept comming on, without the trailer and so far no warning lights.
Thanks again Snow...
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