1990 Tracker Constant 12 volt to stereo

Having bought the wife a much needed new stereo system, and the required harnesses to connect it, I seem to have stumbled into a problem.

The head unit would not power up at all, and after sitting in the vehicle for about 5 hours trying to hidden telepathic powers to bring it to life (read as, just staring at it thinking "Why won't it work, Why.. WHy... Why...... Work Dammit... Work" and a slew of vocabulary that I'm sure would have me removed in an instant) I remembered something about Sony head units. They don't work if there is no line to the battery for memory purposes.
I bridged the ignition accessory power with a bit of foil, just to see if the unit worked, and sure enough it sprang to life.

So I'm left wondering, if there's something simple I can do to have power coming from the battery direct to the harness, without running a new wire off the battery and botching it into the harness. (Ie, is there a fuse to the direct battery connection, or a diagram showing where I need to look, to make sure this is connected properly?)

There must be some way I can get this one wire back to life without having to try and bring something through from the battery to inside (and preferably something that doesn't involve the dashboard coming off again, as half of it exploded into pieces the last time it was removed, and I fear there won't be anything left to put back if I take it apart again)

Please help.

https://www.the12volt.com/installbay/detail.asp?info=alarm&year=1989-91&make=Geo&model=Tracker&ID=17889&type=Stereo

According to this info, your constant should be the white wire.  Check your fuses with a meter.

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My ex once told me I have a perfect face for radio.

Thanks geepherder, unfortunately I already looked at that page, and my wire does not share that colour. I know which wire at the harness does the job, but it's dead, and I was hoping to get some info about where that originates, so I can see if there is a point where it is still live, and replace the dead section, without having to yank off the dashboard. Hopefully there is a fuse inline with it somewhere, which has blown, but I'd rather know where to look than pull the car apart.

Thanks again anyway