Good time to clean up the Climate module
Just finished that job myself. Since it is so much work to get to the Climate module that the heater core is in, I also took the A/C evap core out, put saran wrap over the individual fittings wrapped with rubber bands plus another layer and more rubber bands and then carefully cleaned all the crud caked on the blower side of the fins. Then I took it inside and soaked in a sink full of hot water with about 1 cup of clorox (kills mold and mildew). Was careful not to immerse the fitting ends. Then rinsed it well and soaked it twice in clear water to get as much of the clorox smell off as possible (still smelled like a Laundromat for a week or so). Removed everything from the plastic housing and washed it well with soap and water. It's easy work, just time consuming but worth it because no worry about the musty smell for a long time because its as clean as from the factory.