I installed a 7.5 amp fuse en route

in route to what , from where ? between starter relay C4 and ? ... the Boyer white
( with the wire from the kill-switch connected ... at the Boyer white also ) .

C4 should only be hot when the start button activates the starter relay .
the fuse needs to protect against overloading the kill-switch ( in a couple of unexpected scenarios )
( there is a possibility for current to go both ways when the start button is pressed )