The overvoltage relay works like this. B+ (12v pos.) enters at one end of the fuse, at the other end power is distributed to consumers such as CIS, However the downstream end of the fuse also has a connection to a Zener Diode (Zener diodes do not conduct power until until a threshold value has been exceeded.)The other end of the diode goes to ground. If the voltage in the ssysstem exceeds the threshold the diode switches on and creates a short to ground and blows the fuse!
The wire that brings power to the relay is a 2.5mm cable and can easily withstand a 25 amp surge.
Check your battery for correct level.
Mike