If the OP did that, the indicator LED would always be lit, which would not be helpful. If he connected the LED to the feed from the touch pad to the mirror's LED lights, then it would only be lit when those mirror lights were on - which would be of no use/value.
What the OP wants is an indicator which is lit when the demister is on and, since that appears to be supplied with 230V AC, a neon (on the output of the pull switch he intends to install) would be the appropriate course.
Kind Regards, John
Edit: slightly too slow, yet again!