Kenwood HU only turns JBL amp on part of the time

I have a 2010 Prius with the JBL system. I switched the head unit to a Kenwood and used the AXXESS Metra TYTO-01 harness. Everything worked beautifully the first day, but since then, the amp itself only seems to turn on about 1 out of every 10 times I start the car. It's like it's not receiving the signal to turn on most of the time, but then suddenly it will. It's not a fuse or loose wire -- nothing cuts in and out, and I can turn the car off and on several times while sitting still and one time it will magically work.

I tied the blue and white line for the amp into the red wire as indicated in a YouTube video. (Should it tie into the yellow instead?)

I did not use the Axxess ASWC-1 Steering Wheel Control Interface. If installing that will fix the problem, let me know; I don't normally use the steering wheel controls, so I didn't want to spend the money on it.

I've tried turning it off and on in accessory mode, by starting the car, by starting the car with the parking brake on, and it doesn't make a difference.

I moved the seat and got to the amp, and when the sound was working, I separately disconnected one of the three harnesses from the amp, and when I plugged it back in (car still on, radio still going), the amp would not turn back on. I poked around the pins with a multimeter but couldn't detect any voltage.

I also tried the following procedure someone else posted:
Step 1: Before removing the stock radio, turn to FM and turn up the volume to level 45 (yes, 45.) Set Bass, Mid, Treble, Fade, Balance all to 0 (zero). Let that stay on for about 5 seconds, then turn off the radio, then the car. Now, you can remove the stock radio.
Step 2: Install the JBL harness and the Android radio, but do not turn on yet.
Step 3: VERY IMPORTANT: To prime the JBL system to work properly with the new radio do the following: Turn the car to 'ON' for 2-3 seconds, then quickly turn the car OFF. Do this step 2 times in a row.

The amp worked fine with the stock head unit. Is there some other procedure I need to try? Thanks in advance.