Feel the rubber pipes that lead to the heater unit inside the vehicle. They should be good and hot if the engine has been running for a while. If not, I think there could be air trapped in the system which is preventing the liquid from circulating freely and what we to to fix this is to carefully remove any rubber hose thats at the top of the engine, allowing any air to escape before reconnecting it again...but there is a risk of scalding if you don't know what you are doing. If your engine temperature gauge is indicating cool then the thermostat needs to be replaced. You only need to go looking for fuse problems if the heater fan doesn't come on.
