Oh my god I want to punch whoever did that. Seriously. My very first car, an '85 Ford Crown Vic, came to me like this. Someone had literally sawed a hole in the dash and chopped up the factory wiring, which they then secured to the wiring of the shoddy aftermarket deck by bending the wires into a "J" shape and hooking them together. And then covering that atrocity with clear tape. [spank]
Anyway, one option is that you can get a "reverse" harness for like $10 that will allow you to repair the damaged harness, and restore it to original factory condition. This way, you'll be able to reinstall the factory radio (if it was included) or throw in a compatible unit from a junkyard. In addition, you could then use a normal adapter harness, if you wanted, to then adapt the repaired harness to an aftermarket radio using a regular harness. Just like the previous owner should have done in the beginning.
Option 2 is to do as pcordes1979 suggested, however I wouldn't trust those terrible splice connectors, and just looking at the photo makes me want to reach through the screen and cut them off. The "right" way to do it is with solder and heat shrink, or with crimp connectors and a proper crimp tool, if you're uncomfortable with soldering.
Referring back to the first option, you'd probably want to crimp anyway if you choose to repair the wiring with a reverse harness. Soldering that close to the dash isn't easy, especially if you aren't experienced at it.
If you lived in my city, I'd happily fix it for you. Just buy the parts. Because I hate this kind of damage so much! [gunfire]