In the US, the manufacture has to prove that a failure occurred because of a modification before they can deny a warranty claim. Otherwise, my engine blows up and BMW comes back to me and says, "Well, because of your hardwired V1 we are not going to pay for the new engine."
The onus is on them to prove it.
Another problem. Modifying the car then removing the mod when there is an issue may be fraud. I am not an attorney and I am not playing one on TV.

Now that I have completly hijacked this thread. Sorry
M