OP, you can usually find what the max voltage is on those remote wires (for the amp and head unit side). They are very small, and they usually max out around 500mv on the head unit. Amps are anywhere from 100-300mv (from what I've seen). Some times you can do three amps, some times you can't do two. You MIGHT be able to get the turn on voltage for all three amps, and if you can find your head units max voltage, you might be within spec. If you are not, or can't, it might fry that little switch requiring a costly repair on an expensive head unit, or making even a $100 head unit worthless. You can either piggyback as suggested, or you run a 30 amp relay from the remote wire. The relays are pretty simple, a switch in wire, a 12v circuit, a ground, and a switched circuit out. (+ & - can be connected off amp blocks). If there are things you wanted to add to your setup, like a fan or lights, you can do it this way to have em switched on with the head unit too by using a busbar or stacking connectors, instead of having em always on.
Yeah, definitely not common. Just thinking out loud, but this might only happen with the older style alts. On the older alternators, that had the internal voltage regulators. IF it went bad, It MIGHT overcharge. On the newer alts, they are externally regulated by the ECU (on some manufacturers that I know of), and the ECU, MIGHT be able to prevent that.