Used to be you could trust what the guys at Radio Shack were telling you. Those days are long gone.
While fuses are a must, unless the radio has the reverse polarity diode, it wouldn't have saved this radio. The fuse would pop when it saw the dead short presented by the diode. If there is no diode, there might not be enough current flow to open the fuse, but enough to cook the radio.
As for test lights, they are OK for some applications, like telling you if a circuit is energized, but they won't necessarily tell you the correct polarity. This is where a multmeter and the skills to use it can come in handy. Learning to use one can take some time and practice, but once you know, it can really do a lot. I love my meter, and it's one of my favorite tools. Usually the first thing I grab out of the truck at work when I'm going to repair something.