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The mechanic is not a very good mechanic, as far as diagnostic skills.

Procedure: Measure the current draw on the battery, and its open-circuit voltage, with and without fuse installed, with engine off. Then measure battery voltage at idle. Current draw when off should be about 30mA or so. Battery should be about 12.5V with engine off, and 14V at idle. Radios only draw about 1mA or less of current for backup power, and typically only a couple of amps when running. So if running the car with the radio fuse installed can pull the battery down, it's NOT the radio.

Often there are other circuits on the radio fuse, such as the dome light, glove box light, or even one of the computers.

So the thing to do after the above steps is to leave the fuse in, but unplug the radio, and measure the current draw again. I'll bet unplugging the radio makes no difference. The mechanic needs to find out every circuit that is powered off that fuse, and unplug each device one at a time until the current draw drops. That will indicate what's really at fault.
 
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