The obvious answer would be something's shorting out.
Does it blow instantly or after a while of using it?
The former would indicate a dead short, while the latter would indicate
something's heating up and overloading the radio.
Check for loose connections at the radio (or worn wires), bad connectors, and
see that your speaker leads aren't crimped in a way to cause them to short.
(Thereby shorting the radio out.)
Without much more detail, that's about all I can suggest.
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