There's only one plug that's physically plugged into the blower motor and w/o a wiring diagram handy, I don't know wire colors but at full selected fan speed, there should be 12 volts there if the rest of the circuit's OK and yes, less voltage at lower speeds. If MCC, there is an old fashioned type resistor in the housing, lower left of blower motor but if ECC, it's a larger unit with two plugs and several wires. The ECC blower resistor costs about $110 so you don't want to guess at that as it's a non-returnable electric part.
It being winter (at least here now) and being that you might need to drive the car with heat, if it's not obvious you might be best off by paying a dealer a little diagnostic time to get it repaired ASAP. You could order a wiring manual for your car (800-25-volvo or online) but that'll take days to get.