83 300sd blower motor can't turn on at any push button position: measured voltage at fan clip leads (leads disconnected) is 12V, measured voltage at fan with clip leads attached is 0V, jumped fan to battery (clip leads disconnected) - fan runs great. Please advise.

 

Hmm unless you've got an intermittent fault, it sounds like the power going to the blower has some sort of resistor in series that is blocking the current to the blower as soon as it is connected - but if that's the case, there would be at least some voltage across the blower - try setting voltmeter to ever increasing sensitivities and see if there's any voltage at all when the plug is connected (ie when the supply lead shows 12 volt when disconnected from blower).
you could also try connecting a tiny lamp to the supplyleads and see if it glows at all.

Then you could try a 1 ohm resistor (to protect your meter)in series with your 10 amp current setting on your voltmeter - if nothing try it without the 1 ohm resistor. If you don't have a resistor, just start by flicking the connections together at first to see how the meter reacts - that won't hurt it. Then you'd know the amount of the resistance from V=IR - ie resistance = 12/(current in amps)
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