My 1990 Honda civic's heater/AC fan motor has quit working when the car is running. However, when key is on accessory setting (car not running) the fan blows fine (actually only top two speeds.) My radio operates the same way, working when ignition is on, but turns off when motor is started. Anyone know what is going on? Honda could tell me but I don't want to spend big bucks for a diagostic.

Simple problem simple solution. You should not have installed the after-market stereo yourself!

That might be true except the radio and fan have worked fine for the past two years; this is something that just began about a week ago. Now the fan doesn't work anymore, while running on accessories, it just slowed down then stopped. I wonder if it might be the fan switch?

The only thing the radio and fan have in common is location.

They do not share power unless someone has tampered with power in that area of the vehicle.

There are two power supplies to the radio, constant power and ignition power. When the ignition power is off, the constant power consumption is at a trickle.

It seems self-explanatory to me!

Another thought, they may also share a common ground.

I'm exploring the ignition solution as that is the only thing that makes sense. I'm tired of being bored (radio) and hot (AC)! As far as I can tell, there have been no modifications to the wiring harness under the dash. The fan motor has been "off and on" for a while. Probably have to replace it. Car has almost 200k.

Replaced ignition switch; problem solved. Need to replace something on the fan to enable the low speeds to work, but high speeds on AC is OK right now.