overcharging the battery HELP

Typical bosch relay

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87 would come off your starter relay stud, through a fuse/ breaker

30 would connect to the engine bay end of the cut blue wire

86 wouldl connect to the cut blue wire coming out of the bulkhead "N" terminal

85 as shown goes to ground

If you don't mind one last complication, you can run the 85 through a hidden toggle switch to ground, and create a low level anti-theft feature.

I would use no 16 or 14 wire and fuse it with 20A fuse or 15 or 20A breaker