Hi Steve,
A fuse holder is built into the radio. After inserting the fuse a special cover is pushed down on the fuse and twister to the right to secure. This cover is on the end of a heavy wire that can go to the ignition or the amp-meter.
I suspect the problem is in the fuse holder. In the center is a rubber tube insulator. If this insulator is missing you are very likely to have a short and blow the fuse.
I have available an electrical block that provides one side constant on and the other side is coordinated with the ignition switch. It mounts on the firewall and is complete with wiring.
It is also possible that a cold solder joint is shorting out your radio. In that case it needs to be checked on the radio bench.
Let me know if I can help.

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RAY Member Chat Group - Non-Geographical Region
Chevradioman http://www.vccacolumbiariverregion.org/If I had known that growing old would be this much fun---I'd have done it sooner!