Connect PC to car radio speakers

Hello,

I have a sony car radio installed in my sailboat, which is connected to
audio speakers inside and in the cockpit. I now want to be able to connect
my computer to the same speakers, so I can play music from my PC and, even
more important, hear the alarm signals of my navigation software on the PC.
Both radio and computer are at the navigation table.

Since I am not a real technician, can anybody help me in how I should set
this up? I was thinking of installing a car amplifier to which I connect
both the audio out of the radio and the audio of the PC, but will this do
the job? Maybe there's a better solution.

Thanks in advance,
Ben