I have never tapped into the factory harness for any of my accessories. And I have a lot of accessories for an eb.
Everyone one of them runs on a dedicated harness back to a distribution/fuse block.
35 years.
No electrical issues.
My old man insisted back in the early 80's that I install the marine distribution/fuse block we didn't use on our boat. Also at Dad's urging reworked every ground on the eb.
Still have that distribution/fuse block. Still run dedicated harness's ....
My Dad and my grandfather just didn't think the eb's oem harness was robust enough to add an aftermarket stereo, c.b., scanner, c.b. amp, stereo amp, off road lights, etc...
So my advice, from my good experience's, is run a completely separate harness for everything you add including that stereo.