If your regulator and blower (and fuses) check out OK then your pushbutton unit is probably at fault. They are known for intermittent problems. If it is made by Bosch, the solder around the 5 volt regulator integrated circuit cracks from heat. If it is a MK unit (the most common), they develop solder cracks around the switches and temperature adjust potentiometer.

As for your heated seats, what year is your car? Early 124's had the controller module on the floor in the wiring channel - drivers side. They would corrode from the mats getting wet. The controller was later moved to under the back seat. Of course check your fuses first.