Totally agreed.I don't think anyone can answer that question precisely. Toyota has taken a hard line about this. It is their position (apparently) that any modifications to the electrical system are problematic. I tried to get an extra cigarette lighter outlet installed by my dealer on my Prius and was told that Toyota had told them that they were not authorized to do ANY work on the electrical system except to repair or replace factory parts. Another long time Toyota manager told me that they no longer used any local radio shop to fix radio problems as all they were allowed to do was remove and replace the factory radios for the same reason...voiding the warranty.
Now I can see that with all the intricacies of the hybrid system they might be afraid to allow some shade tree mechanic to set up an electrocution, but I think they are dead wrong that ANY touching of the electrical system voids the warranty. What is difficult is that Toyota has thousands of engineers and other experts that are prepared to parrot the party line, and I have common sense and logic. I think I lose!
That is why I only do totally reversible or totally not noticeable mods, at least until the warranty is over. Or mere cosmetic mods, which do not affect anything that really costs a lot of money to fix.