relay for heater blower motor burns out weekly, what is the test procedure

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I have replaced a bad blower motor and the resistor assembly. The resistor was not bad but I changed it because it was old like me. The relay has been failing for several months but I replaced these parts last month and the relay continues to melt it's case at post #87. Is there a chart on current draws for the components? Is there a common problem with this bus. It is a V10 E350 which has an 8 passenger conversion body behind the drivers seats. The conversion is a common conversion for handicap vehicles.