The correct spark plug is double platinum...
AutoLite APP105 is OE design with OE half thread
AutoLite APP765 is OE design but with full thread
How important the thread is, I don't know....I suspect either plug is fine.
This information is from the Rockauto website.
It IS important to get the correct double platinum plug.
Stay away from any multiprong or performance plugs as they can cause problems.
Sticking with OE design is important.
Coil packs are a common fail item for the problem that you are having.
The coil pack is a block that contains the 3 coils, follow the spark plug wires, usually up on the driver's side near the windshield.
No heat is often caused by a issue with the electric motor/nylon gears that drive the "blend door" that moves to permit more or less hot air to mix into the air flow.
The ticking sound you mention last, could be from the nylon gears of the blend door actuator skipping as the electric motor turns, trying to move the blend door.
The pictures that I have are from my 1996 windstar, It did not have the electric motor method of moving the blend door.
Many have removed the trim panels below the windshield in order to reach the rear spark plugs.
I would drive mine up on ramps and reach up from under the vehicle.
Plug wires would also be a good idea at this age of the vehicle if they are original.