Put in aftermarket stereo and used aftermarket wiring harness adapter... won't work unless connect battery and ACC wire together? Draining battery

The positive wire should go to a fused permanent live ie effectively directly to the battery.

The ACC wire should be fed from the key switch and should go live when the key is turned to the 1st position ie accessory.

If the ACC supply from the key switch is faulty then the stereo will not come on.

if you feed it from the ignition position ie position 2, then the stereo will work but only with the ignition on ie engine running.

Solar Mike's answer is correct in it's description, however I would also add the following from experience.

When I have done this in the past (20-odd years ago I think!) the harness adaptor came wired with the correct colours and pin layout according to the ISO specification and the majority of cars but if I remember correctly the permanent and switched (ACC) lives needed to be crossed over dependant on application - i.e. some car manufacturers swapped the two pins over compared to the ISO specification.

If you can get a wiring diagram for your vehicle you should check that the wiring on the car side is what you expect it to be.

A quick indication though is that if the radio can only be switched on and off using its power button and the key position (or even if there is no key in) makes no difference, you have the permanent live connected to the ACC. If the radio loses all it's settings whenever the key is removed then either the switched live (or nothing at all) is connected where the permanent live should be.

Also, check with your stereo manual - it is not clear to me just from the naming whether the Power Const or the +12v pin needs the permanent live connection. I would normally say the +12v, but the Power Const name kind of implies that it should have the constant feed.