Which empty fuse slots are wired to the ignition.

That depends on which optional equipment was installed at the factory. Very often unused slots aren't labeled. All you have to do is probe them with a test light and see which one(s) turns on and off the way you want. Some will be live all the time, some will only be live in the "run" position, and some will be live in the "run" and "accessory" position. Often there are blank single terminals provided for the things you're doing. GM used to label them "Ign" for those that were live in the "run" position, "Acc" for those that were live in "run" and "accessory", and "Bat" for those that were tied right to the battery and were live all the time. Chrysler and Ford usually didn't label the extra terminals they provided. You just had to find the right one with a test light or voltmeter.