This is/was one of my biggest complaints when I was a lead mechanic and had
people working for me. It boggles my mind at the indifference the majority
of mechanics seem to have on proper jumping techniques. Not that you are
indifferent, don't mean to be ragging on you.
I'd say you're lucky to only have this small of a problem after reversing
the cables. The MAX fuse is supposed to protect everything internal but
sometimes isn't fast enough and some things like battery, alternator and
charging system wiring get damaged as they are a direct connection. You
might be lucky and have a fuse in the audio unit that blew but the problem
most of the time is the current flows into the ground side of all the
electronics and digital isn't very tolerant of this. Also, most electronics
aren't protected on the ground side. All you can do is best guess on any
other problems, mine is if it hasn't shown any other problems by now then
there shouldn't be anything lingering. On the other hand a shock to a lot of
electrical components can shorten their life. I wouldn't worry unduly about
it and just try to get audio unit fixed. If you remove it yourself you might
be able to get a "pass/no-pass" test done for little or nothing at an audio
repair or it could be something inexpensive to fix like a polorized
capacitor in the power supply.
HTH, davidj92

HI.

       I hope you had a few words with the guy.!!!!!!!!!!!!!!!!!!!!

The other point is some equipment is fitted with " IDIOT DIODES "
Honest that's the term.
If wrong polarity volts are applied it can pull down the in line fuse fast.
My hobby is Ham Radio and lots of the Jap pieces of equipment have these
diodes
as protection.

So maybe look at the fuse in the car or there maybe be an internal one in
the CD unit.

Just a guess,

Cheers Johnny G3LIV UK.

I don't think they design to protect against a polarity reversal.  The fuse
is there to protect the wiring and car, if the radio has an internal short,
and prevent fire within the car.

Is the radio the only thing that is not working?
If so, you are very lucky, most times a reversal can blow out other boxes
like the computer, etc.
causing $$$ of repair.
Verify everything functions.  there could some units that have been
stressed and will fail later. Search this site for other cases of a
battery reversal connection. I can cause major damage.