The diode should only cook if there is a battery or alternator polarity reversal combined with a high amp draw. If you can disconnect at least one end of the diode from the circuit, you can test it with an ohmmeter. Start on the hi scale and work down to lower scales until you get a mid scale reading. A good diode will read a high resistance with the red and black meter probes on the ends of the diode leads, and a low resistance if you reverse the leads. Usually when they blow, they either crack or look and smell burnt.
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