Quote:

The term switched is typically used to indicate power on with the key in run or accessory position, power off in the key off position...



EXACTLY

But furthermore, how much power do you need for those two lines? If this is a fairly large stereo, probably the "constant 12V" lead is the "main power" to the stereo, and it's important that it be heavy enough and properly fused

The "switched" lead is probably a control lead. What about your AM radio connector? You still using it? It will already be marked "radio" on the fuse panel, so that makes good sense.

The connector should have two leads. One is for the lamp, the other is "switched 12v in accessory"