Yep, done all that. I've used a screwdriver, and I was able to get a #8 power cable through that i'm using to bring a battery ground in behind the HU.

The plan was to use this #8 as a fish wire, to pull the 1/0 through, but it just won't budge.. and I can't see enough to get a blade in there (plus the oem harness goes through this same grommet).