If dad is responsible here, I'd go cheap and effective. Do the amp bypass so the speakers receive good clean signal from the headunit. All you need to do is run 8 speaker wires (2 per channel) from the headunit to the amp location under the seat and splice a power wire for the headunit at the amp plugs. There is a recent thread on the bypass.
The 'amp/signal' ground is the shared (-) output for each speaker channel. The metra harness combines the shielded grounds for each amp output into one, while the (+) output is split 4 ways. The amp itself is electrically grounded to the floor under the seat. The radio shares this same ground in the factory configuration.
I appreciate your input from your experience.
I'm not sure how "responsible dad" = speaker outputs of a cheap Chinese head unit, but it is always an option.
I see that some other Toyota forums report noise issues as well, with the Metra 70-8113 or Scosche TA03 when retaining the OEM amp.
It seems that the issue is not so much the quality of the Metra/Scosche harness, but that the OEM amp expects a different input than the typical aftermarket amplifier.
A relatively simple solution appears to be the addition of 4 resistors:
Scosche TA-03 Wiring Kit Noise Mod
Now I'm curious to see how the current Kenwood is connected to the system (installed by a 3rd party), as we haven't had any noise issues with that unit. Is it running through the OEM amp or is it directly wired to the speakers?
The current plan is to install the head unit when my son arrives home from college for summer (May), and have it functional with minimal changes. Speakers/aftermarket amp/sub(s) to be installed over June/July. It's a stereo upgrade, but it's also father son project time. If we run across a few issues, it will provide some teaching opportunities.
Again, I appreciate the heads up on the noise issue, as it will allow me to be a bit more prepared to deal with it.