Before you replace your heater, make sure you don't have air in the cabin-heater coolant loop. I suppose if you turn on the heater and you don't see the climate power consumption go up on the dash, that could be a sign. If you have Forscan working on your car (OBDII dongle + app) you can see what the coolant temp is at the heater.
When I had my battery replaced, The dealer didn't purge the air out of the coolant lines. The cabin heater is on one of 3 loops. The other 2 loops have de-gas line that goes from the top of the loop to the coolant reservoir, but the cabin-heater loop does not. If air gets in their, you will get no heat.