1991 Honda Civic car heater not working all the time

My car heater works sometimes. When I turn it on to heat my front windows it will either start working or it wont work at all and the power of the heat is low. Its not like new cars with fast heat. I use the heater only to keep my windows from fogging up on me while I am driving to work and home. Like I said sometimes it works and sometimes it dont. Like to know whats causing this. I was thinking a worn out heater.

The part of the heater that does not work half the time is the blower.