So, for a few weeks I have been worried about this same problem, I live in Milwaukee and this is my first Winter with the car. I didn't realize the heater had an issue until the weather recently started getting cold. I have the same problem, and here's what I've learned.
1. Make sure there's no air in your coolant system. If you hear bubbles/rushing water sound under your dash when the engine first comes on, you probably have bubbles in your system, good news! This is an easy fix.
2. I took my car to a local mechanic for a proper flush of the coolant. They backflushed the heater core as well, so I know it's clean, and I don't have any air bubbles. My heater is very weak still though. It gets decently hot once the coolant reaches ~190 or so. Full operating temp. If the coolant temp is below 150, there's supposed to be a supplementary electric heater that kicks on. I suspect mine isn't working, but idk for sure. Judging by the diagrams, the heater core/electric heater are as inside the dash as it gets.
3. The flow of air out of the vents is way higher when the climate control is set to "Lo" or 60. When I raise the climate control to "Hi" or 85, airflow goes down considerably. The blower motor doesn't slow down at all, airflow only goes down. This makes me think there's something blocking the air at the heater core. Does this make sense to anyone else?
If anyone has any clues, I'd love to hear about it.
