1989 B2600I wireing

Hi, Every body, I am new to this forum. My question is,, I have a 1989 B2600I some one jumped a wire through a relay to feed 12 volts to the black wire with white tracer at the test plug near the wiper motor. What could be the reason for this? When I disconnect it the truck won't start. Thanks for any info.. from jerrywf

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Is it coming off of the 30a head fuse? I think I have that same problem where's the test plug for the wiper motor at?