If you just cut the connector off an s5 car and add a s4 connector to it will that cause problems? I know the s5 sees voltage all the time and the s4 only when the key is on, but I don't know the whole theory on how they work. I've got an FD in my s4 with a wire added to the battery and have had no problems. But my brothers car has went through 3 s4 alternators when his s5 one died, one of which was my original T2 with only like 50,000 miles when I changed it at most.
It could be a coincidence but maybe it's burning them up being powered all the time?

 

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