Seeking Stereo Replacement for '89 100Q

["Michael D. Callaghan" <mcallaghan@earthlink.net>]

I just bought a 1989 100Q. The car seems fine in all respects (so far),
except for the cassette deck. It doesn't work very well.

I have no desire to rip the car apart, and I've looked through the list
archives for "stereo." I found much useful info, but I'd like to summarize
my desires here, and see if anyone can make a specific recommendation.

I want to replace my head unit with a 3rd party unit that has a cassette
deck. I don't want to replace any of my factory speakers (I do not have the
Bose system), or run any more wires, if possible. 

I read in the list archives that Crutchfield sells a wiring harness that
will convert a head-unit's output to be compatible with my current setup.
Is this indeed accurate?

I want to buy a head unit, pull out the current one, and replace it. Just
like that. Is this doable?

Thanks,
MikeC