On all circuits, the load is drawing(/pulling) the power it needs, (it's not the battery pushing). So where ever you put it, those switches are very little power (usually only around a few hundred millivolts or less), and just pulling what they need through the circuit. As long as the circuit prior to the new load is not overloaded, then no problem.
That remote wire is a on/off switch. Your main power has the juice, but the remote power wire is the on/off button. If you connect that to a main power wire, it will always be on. It will always be drawing, and draining your battery. It really should be connected to the remote wire off the head unit, because the only time you need amps is when the head unit is on. If you think of it like your tv, and it didn't have a plug. It was hardwired into the wall, that remote wire is the power button on your remote. It's the easiest way to turn it on and off, you COULD wire in a switch, or you need to undo the wiring to shut it off. When wired in to the head unit, it's a "hands-free power button" that you CAN'T forget.
If you're testing it no problem. You could install an inline switch to control it when you want to, but if you just run that switch off power: 1. How will you turn it off? 2. Even if you add a switch there is a huge potential to forget and kill your battery.