My heater has slowly lost heating efficiency. Now it only heats the car when I am driving...it does not heat at idling. Is this the heater core? Or perhaps some easier issue? Any help would be appreciated

The heater core could be plugging up. You could try back flushing it or use a cleaner of some sort in the coolant to see if that helps. I assume the coolant level is ok. One other possibility is a head gasket leak may be blocking the coolant.

i have a 86 sedan which did the same thing , the thermostat was bad , [ closed] and the temp gauge would not even go a 1/4 of the way up even in the summer , changed it and it was fine

yeh I second that the t-stat is the issue. Does it take ages for the engine to heat up? The thermostat is located on the right hand side of the inlet manifold on ea81s, basically its under the housing where the top radiator hose goes. Make sure the t-stat hasnot fallen out of its notch before you bolt the housing. You should also use a new gasket.