If the engine is still out of the truck you shouldn't have to pull the heater blower unit out to flush it. You could get a funnel, plug up the outlet heater hose and fill the heater core with citric acid solution and let it sit for a few hours then dump it and reverse flush with a garden hose. Ideally it would be good if you had a way to circulate the solution through it like I did as it will do a better job (got an old fish pond pump somewhere?)