This article includes details on making a simple volume control box for when you want to hook up an MP3 player or similar source to an car amplifier without a head unit.
Basically, you want a way of limiting the amount of signal getting to the amp, you could use the built in volume on a MP3 player, but this will be a bit course and in general isn’t great for adjusting while driving.
So, you need to add a pot to each channel, the second picture shows quite nicely a dual gang pot, which is just 2 pots with a shared knob.
The second part is controlling the power for the amp, car amps have a remote power line, taking it high (to 12v) will turn the amp on, and making it open circuit will shut it down. All you need to do is simply put a switch between the blue wire on an ISO connector and 12v, this can be ignition switch or constant. You can also easily fit a LED, just put it between the remote side of the switch and ground, with a 4k7 resistor.
One important thing to note: Do not connect chassis ground to audio ground, as this will generate some pretty amazing hum!