I think we’ve got off topic , the problem is that there are gremlins in the dash and eps

 

Off topic? The problem is figuring out what a bunch of clowns might have done. If you know which wires are supposed to be connected to what you can look and see what is wrong. And just maybe get it working.
I have a feeling they might have tried to get constant power or ground off of the power steering lug on the fuse box.

 

Very possible, so let the wiring harness diagram you provided give an inkling of direction to unraveling the problem.
for future readers, please don’t try to cut into the wiring harness, thinking old school methods to connect an aftermarket head unit! What worked in an ‘80’s car won’t work in the HHR without the correct modules and adaptors. I’ve had to go to a few places to help restore the integrity of the wire harness to connect to the HHR . I had to get a replacement factory head unit for my own HHR after a battery replacement. It’s frustrating as it defied all old school logic!

 

Any chance of undoing what was done?