I run into this question all the time with customers on thier vehicles, only correct answer to give them is none at all, but to throw some things at them to make them think.
what is the vehicle used for?
what would you get to replace it if you were to go that route?
will the replacement of the vehicle drive your vehicle insurance and license and registration fees through the roof and lets not for get sales tax and personal property taxes.
little things like this are what they all seem to forget when they say, " screw that, the car isnt worth it to put that much money into" they usually havent stopped and thought about the other side of the coin. not to mention in your case you will have more time invested due to having to rip out the audio and put it into another vehicle.
in the end, you have to make the decission for yourself, but you need to stop and think about what is that car worth to YOU, not just is it worth putting that kinda money into.
other things to think about, you probably know some people that work on cars for a living, find one of them that you know is dependable and has free time on the weekends and wants to make a little extra cash. most techs will work for 20-30 bucks per flat rate hour compared to 80 -120 a flat rate hour that a shop charges and they usually wont nail you as hard on mark up on parts as a shop will either, hell if ya know the guy well enough they might even give ya the parts at the same cost they can get em for.
hope this helps
Chris
Automotive Technician/Service Manager