12 volts directly to the blower motor will make it spin providing the motor is good, but how long it would work depends on the current capacity of the bypass circuits.
From the description of the problem it seems there is a wire connection problem somewhere possibly in the return side of the blower circuit. A bad connection can heat up when current flows through it and that makes the resistance of the bad connection go even higher and the motor slows down even more as time goes on.
A lot of blower motor circuits are supplied power via a fuse and relay to the power or HOT side of the blower motor. The ON-OFF switch and speed control of the motor are on the return or GROUND side of the motor circuit. If that is the design yours is then using a good ground reference point for a meter, check the voltage at the motor while the blower should be running at full speed. If you have close to 12 volts at the motor and the motor is running slow or not at all then the supply side of the motor is good and the trouble is on the return side of the circuit. You will see that there is a pretty large voltage on that side also because of a voltage drop happening further down the return side of the circuit. Ideally, there should be no voltage on the return side when the motor speed is set to high since that is the ground side of the circuit, but there will always be a little there even if things are fine due to wire loss. At slower speeds resistance is added which causes the current to drop and that slows the motor speed down. Look for signs of burning at the connection points indicating a bad connection point.