High level is the wires that go to the speakers.
If you used those I can't imagine why your volume control won't work. Only time I know of that is when using certain optical, digital dsp's or amps.
Now if you used the signal going from the radio to the stock amp maybe your volume control controlled the old amp itself and not the signal.
I'm sure someone that knows a lot more will chime in to help you.

 

I'll put more detail about my install in this reply. Thank you for your response though. I am installing this for a friend and I got everything working but the volume control. I must also include that the fade and balance don't work and neither do the treble and bass adjustments. The head unit is a touch screen mark Levinson head unit with navigation.

Here's how my install has gone so far.

I cut the wires going to the old harness for the old amplifier. I spliced in my high level input adapters in the the signals coming from the factory head unit. I then connected to the speaker outputs and connected a line level converter with a bass knob, from the new amp to a sub amp and subwoofer. I connected the ground, constant and remote and set the outputs to full on the amplifier. I turned the input signals to half way (gain) and everything sounds nice in the car. The only way to adjust the volume is to adjust the input signal knobs on the back of the amp for channels 1&2 and channels 3&4. The subwoofer gets signal and so do the speakers. The speakers and sub have full signal and play loud and clear. Everything sounds good but I cannot adjust the volume, fade, balance, or bass and treble on the touch screen head unit. I can turn the audio on and off, so I can cut the signal to the amp, but I cannot adjust the volume of the signal. I could expect to lose the fade and balance adjustments with an aftermarket amp install, and maybe even the bass and treble adjustments, but I don't know why I cant adjust the volume.

The volume knob works fine, I even took it off and moved the pin itself to make sure. It did in fact work before I installed the new unit. I am about to replace the speakers tomorrow, but I doubt that will change anything. I feel as if the signals are fixed, and the factory amplifier was the one doing the adjusting, so I need to figure out a way to be able to restore the ability to adjust the volume if that's the case. I am wondering if connecting loc's or some adapter would maybe change anything? Would rca's correct the fixed output? Or is this not a fixed output problem?

I feel like somewhere I saw someone say that the radio has a volume control wire lead to the amp, but I can't find the website I read that from.

If anyone has ANY advice as to how to fix this problem, please let me know. Hopefully I am missing a step.

 

You need to tap your signal after the amp (factory amp) and not from the radio. The radio controls the volume at the amp, likely with data. The signal from radio to amp is straight fed and the volume is controlled at the amp.

What year is the vehicle?

 

It's a 2004 Lexus ES330. The factory amp is blown so there isn't a way to connect it. And I don't would think running the old amplifier with a new speaker amp would work? If all else fails I might just run a level knob or "bass knob" from the speaker amp to control master volume. I found a review for one of these aftermarket knobs and the guy said he needed to control his volume too and it worked. It's definitely not as clean and convienent as using the stereo to control it but if it's the only thing to do, then I might have to. Let me know if you have any other ideas. Thanks

 

You're on the right track. You may have to go down that route.

In some vehicles once those factory amps go kaput all hell breaks loose.

 

Being that all control is gone I'd be trying to talk any one into a dsp. This would be an ideal situation for one.
Not only tuning abilities but also almost all dsps have a volume control. Some come with it , some are sold separate.
Unfortunately it was what I feared. The radio simply controlled the amp that had a very basic dsp in it.
Seems like all the new factory upgrade systems are doing this these days.

 

mark levinson system controls factory amp, without factory amp it became exponentially difficult to control audio settings.
you can replace head unit altogether or find non blown factory amp,(shouldn't be too difficult for 13 years old vehicle).

 

This was my next thought.

Crutchfield lists the dash kit at $25, throw in a harness, half of which you'll use and half which you'll have to bypass to the factory speaker wiring and get something new or used for the dash.