This is going to come off as rude, apologies in advance.
Alternator whine is related to a bad or inadequate ground 99.999% of the time. Ignition circuit shielding could be an issue but that is extremely rare and more of a 50s & 60s car thing.
CHECK YOUR CHASSIS GROUND.
This is why I stressed GOOD ground in my last reply to you. That not only means good contact to metal, but also properly sized wire, and good chassis ground.
From my 71 Friebird, to my 80 Camaro, to my mid 80's camaro RS, late 80's IROC, mid 80s Lincoln and Grand Prix, mid 80s Nissan hardbody, 94, 96, and 99 Ford Ranger, current 93 Trans Am, 04 Toyota Tundra, and new 2018 Hyundai Elantra, and countless installs for friends, I HAVE NEVER HAD ALTERNATOR WHINE NOT RELATED TO GROUNDS.
I also never bought cheap amps. My current (bought in 1998 for $550) MTX Thunder 225HO (25wx2) actually puts down nearly 1,000w at .5 Ohm and is .25 Ohm stable. Used factory alternator with it in 94,96,99 Ranger and 93 Trans Am, never burned an alternator out with it either. It's power wire and ground is as thick as my thumb.
Of course, I also said NOT to hook into the cars electrical, so...
...run the 12v constant to the battery, not anywhere else.
...tie the 12v switched to the 12v constant with a toggle switch on it somewhere so you can turn it on and off.
...verify good ground at black wire AND your chassis.
That's a bottom shelf unit. Probably Chinese trash with an "American" sticker on it. I can tell by looking at it that it probably puts down less power than a $50 head unit. Want to know its true power? Multiply its fuse rating by 12 then 0.60. AT BEST, thats its true output, and 60% efficiency is probably being kind.
You can add resistors which suck power. You can try sheilding. You can do a bunch of things that are not required if the wiring is correct.
So, again, DO NOT TIE INTO THE VEHICLES ELECTRICAL. Professionals never install amps, crossovers, eq's, etc. Into the vehicles electrical no matter how tiny it may be or how little power consumption it has. Guess why. Sure the head unit is tied in and normally with an installation harness so factory wires are not cut or altered.
You can use the same constant 12v source for both the power and turn-on on that thing. Just splice the two together at the end and install a switch inline with the orange wire as previously stated.
So, red wire from battery to 12V input on amp. Orange wire connected to red wire. Cut orange wire and add a toggle switch to it.
Do you have a radio? Seeing the inputs being a 3.5mm headphone type I assume you want to plug the amp into your phone?
If you have a cassette player, they make $10 cassette adapters that plug into your phone headphone port to play thru the tape deck. For $50 you could have a stereo with bluetooth and forget the wires. Either option is better than just some cheap amp, but if you insist on this amp, and splicing into your electrical then chasing grounds and ignition system isolation is going to be mandatory.
I agree with your vendor 100%. Of course, something that cheap might be defective and have a bad INTERNAL ground, but, good luck finding that or getting vendor to own it. We get what we pay for.