I have a 1990 B250 conversion van. When I switch on the ignition, siometimes the a/c blower and power windows work and sometimes they don't. I checked the wiring diagram and the two seem to be connected but where I don't know. Could this be the ignition switch or if not where else? I'm tired of switching the engine off and on to get the blower to run. Once its running it never stops and if it does run at first, it never comes on. It's either on or off each time but intermittent.

 

Short of tracing the power loss when it happens, your first option would be to replace the ignition switch.

 

I've had this problem in the summertime. On mine, it was simply the fuse in the fuse box located in the glove compartment. It could also be that the fuse holder is sort of melted and the fuse although good is not making a good connection. It is the second or third fuse from the right. For some reason, they are both on the same circuit.

Hope this helps!!