So you've made sure that both hoses on the heater core are hot? If one is significantly colder/cooler then the core is plugged badly. You can try disconnecting the hoses and backflushing it with high pressure water. May help, may not.
If both hoses are indeed hot...then yes, your most likely suspect is the actuator for the temperature. It's the one located roughly middle of the dash behind bar that separates the stereo and glovebox area (88-94 models.)
In this pic with the dash removed, it's in the middle of the black HVAC duct box, my example has the yellow/black sticker on it that stands out very obviously. Yours may not have the sticker; the idea is just to show you the area.
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( Also to give you an idea how it operates, the airflow is moving sideways from the fan on the passenger side, through the evaporator core which stands straight up in that area with the hump at the top. The heater core lays sideways below all that, and there's a flap in between that can either force all the airflow down through through the heater core, or allow a mix depending on how far the flap is closed.)
With dash installed, you can still just barely see it peeking out -
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You should be able to pop the center vent/stereo bezel assembly out, and the glovebox compartment out, and be able to sneak in and get this actuator removed. The good news is this one rarely goes bad so if you don't want to lay out bucks for a brand new one, you could gamble on a junkyard piece if it's cheap.
Richard
