96 heater-AC fan

The fan the the van kind of runs when it wants to. If the fan switch is on it might come on. Then also at time when it is runnibng and I turn off the ignition switch it will stay on until I turn off the fan switch.
Where do I look for a short?? If I hook jumpers to the fan plug it will run that way and work fine. Any advise will be a help.