There is a spot for a "radio" fuse as described above. It's the top left fuse and it was used for a very low power draw and simple 1970's radio. It takes a 3 Amp (short) glass fuse but it looks like your fusebox is not populated with the terminals or the jumper... In my case, I raided another fusebox to grab more terminals and jumpers to populate mine with a 'Radio' fuse and also a Rear Window Demist fuse, because I wanted things to be marked similarly to the factory.
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See the jumper that reaches from the RHS of the fusebox circuit to the 'radio' fuse supply. (tan coloured wire)
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The "Radio" circuit has the same active status as the heater fan... which is that it gets power with ignition on, or accessories. This is basically the "ON" signal for your aftermarket stereo.
Regardless of which you use, the best practice will be to use this to not actually power the stereo, only to tell the stereo that when it has 12V, the stereo should be on (and when the 12V drops to zero, the stereo should turn off).
But your stereo will also need a constant 12V supply so that it doesn't lose its presets etc.
Additionally to that, if you used the standard 3A 'radio' fuse you wouldn't have enough current available for a decent stereo that actually has some grunt. Amplifiers, for example those driving decent power speakers or a sub are going to draw a lot more than the original stereo. I would run my own (fused) constant 12V supply from the battery or alternator for that purpose. It should not need to be switched on and off by the car.
I have a number of 12V constant power wires that connect to a firewall power post I installed, and one of them covers headlights, one covers stereo, one covers the main body loom (via the standard fuse box) engine management.
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Your aftermarket air con is probably powered in some direct way like this.
The heater fan supply (rated at 15A) if no longer in use, will provide a higher current ceiling (15A rather than 3A), but you'll lose all of your headunit settings (clock, display colour, what you were playing last) every time you turn the ignition off. So I wouldn't say that's a solution. If you were lazier, you could tap something off the main red cable that feeds the whole fuse box, but I wouldn't recommend stressing that original wire any more.