The fuse is doing its job and probably preventing a melted
wire (or worse). So, you need to find the source of the electrical
short. I suspect the red wire you see is one someone slipped
under one end of the fuse as a 12V power source. The good news
is they apparently placed it on the correct side so the power
is fused (if it is on the other side of the fuse then it would
probably have melted by now). I suggest you find out what the
red wire is for. Try disconnecting the red wire and see if the
problem goes away. If it does, there is the problem. If not,
then the radio is probably the problem.

Do you have any idea what the red wire is for? Is the radio a
stock radio or after-market (possibly with an amplifier somewhere)?
Any after-market fog lights or other lights?

If you don't know volts from amps then maybe have a car stereo
shop have a look at it. They typically deal with adding after-market
equipment and often see jury-rigged setups like this that need to
be cleaned up a bit. Visit during a weekday (when they are slow)
and they might be inclined to only charge a minimal fee.