Just got myself a kinetic 7001 DAB/FM amplifier and it needs a live feed when ignition is on. The instructions below say:
"The supposed +12V switched output from many radios is rather low.
So, this amplifier that you've bought is an aerial amplifier. The output from the radio is for driving a powered aerial or for an electric aerial (in days gone by). So, you'd expect the radio output to be able to drive this amplifier, but the instructions of kinetic amplifier ought to know better than I do what the drain of their device is.
However, do you ever use the radio when the ignition is off? If you do, you'll need either a supply which is switched with the radio, or which is permanently on. If you use a supply which is permanently on, then you'll have the permanent power drain, which could be an issue if you leave the car for extended periods without running it.
If they've suggested a 0.5 amp fuse, that is one upper limit on the current draw. The current ought to be way less than this (do they quote a figure?). You can multiply this by the number of hours that you might leave the car without running (remembering holidays, and at the airport, etc) and if this is a small percentage of the battery capacity, it probably isn't a big worry.
If you need to switch the amplifier's supply, and the output from the head unit doesn't do the job, you can get there with a couple of transistors, or something.