They do get electrical gremlins at that age. 

Radio can cut off after opening the rear vent windows. The dome lights can remain on after vehicle is off.

Maybe something in the ignition switch or relay sticking?

There is more than one problem there... guaranteed. 

The IGNITION SWITCH is probably bad in that vehicle. There was a service paper on it as well as a recall on bad switches. Some switches not covered were bad also. CHANGE IT. 

Bad grounds on the engine vs the body are a problem in older trucks and cars. Make a trifecta of ground. Heavy ground (like a starter wire) from the neg of the battery to the steel in the fire wall. From the battery to the engine block. From the battery to the frame. 

Then run a ground wire from the engine block to the body on those connection points. Eliminates differences in ground potential in old rusty cars and trucks. ** Generally the spot welds carry the bulk of the power for ground in the body of cars and trucks. Metal fatigue makes those spot welds electrically crap after so many years bouncing down the road. You have to sure up the electrical path of ground with some new heavy cables and the BS will go away. 

My guess is that much other stuff is going to ground through the blower motor as well as the ignition switch is misbehaving. Start with those two suggestions and come find me and tell me what is still going on. I will have to do some tests and we will find diablo which is hiding in the wires somewhere and kick him out LOL

Sounds like you are creating a phantom circuit when the radio works in Park and the radio is most likely getting a ground through neutral switch wiring, meaning that the radios ground is compromised. Electricity will always take the path of least resistance. The blower motor continuing to run after the key is turned off may be as simple as a stuck blower motor relay.