It could be a number of things, from a loose or bad connections, could be blower motor resistor, or it could be the blower motor itself or worse yet, the HVAC controls. Resistor is built into the blower motor assembly for the front, i would start there, roughly $170 in parts, and generally a pretty easy job, here's a video, i haven't watched it, but i've done my fair share of blower motors, and they are generally a 5-10 minute job. the one in my 05 was honestly my favorite, stupid thing just twisted in, didn't even need any tools to swap it out