How high a level are "high level inputs" on an amp? Are they for head units that don't have low level outputs so you can use the speaker outputs?
I ask because I'm trying to use an OEM radio with an aftermarket amp, only to realize that the wiring makes no sense to me, and that the antenna probably wont work. I've looked it up, but there seems to be nothing for 92+ ES300s, and a lot of links are dead.
The wiring diagram below shows some of the four inputs to the factory amp, but where are the corresponding grounds?

Could I leave my factory amp plugged in and use it to feed audio to my aftermarket amp?

 

You need a line level converter mate. If you thinking subs that is.