You can usually slip a probe into the connector along side the wires w/o unplugging it. You can also use a good ground for the Blk...or even a test lamp probe will do. Your main concern is that you have 12v at Red and a voltage sig at yl.
The firewall plug is power feed side and the connector/wires goes to the blower/regulator, so if you do unplug it , you check at the plug that stays on the friewall.
If you have power at both R and Yl, with blk being used for ground on both test, then the ACC supply side of the circuit is OK and the prob is down at the blower..meaning you then have to dig down in there ..that is why you do the test at this convienient test point first. You do not want to go to the trouble of dismantle to get to the blower/reg , only to find out the problem is Upstream......
Many a DIYer checks the fuse and then decide it must be a problem down in the blower compartment, only to find out there is no variable V at the yellow wire..which is the trigger signal to switch the regulater ON..w/o that sig, you have NO Blower ...so that is as important a test as the Red 12V one.... an often mis-diagnosed condition.

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A Dalton