Secretbuzzard

It’s almost certainly going to be the blower speed resistor board on the side of the heater box. After years of sitting in the airflow to keep it cool the resistance wires rust and get thin and then break. They aren’t expensive and if you don’t have a/c are fairly easy to replace. However, with a/c the evaporator blocks access and must be removed! Before I installed my a/c this last summer I replaced my resistor pack with an upgraded type from Rimmer. It lasted about a month and forced me to pull the evap assembly after discharging the freon and recharging after the replacement. I was not a happy camper at that point. The second new one has been working great since then.
Not a fun job. Be very careful with the two pipes going forever through the firewall as the evap swings a bit when coming out and they can catch the edge of the hole.