That plug is feed from the control panel and is used for testing power down to the reg/blower...but the easy way to test a blower motor circuit Current draw is to place the leads of the amp meter across the blower load fuse with the fuse removed.
You can also use a fuse on one lead for protection or leave one end of the Blower fuse in the connector and put the meter in Series with the other end of the fuse and the fuse connector.. Inother words, the meter has to be in series with the blower feed .

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A Dalton