I live in a manufactured home. The furnace is electric and inside. The central air is a unit outside. I can't find a cold air return but there is a flex duct coming out of the ceiling above the furnace but is not connected to anything. Where should this b ehooked up and where does it go up in the ceiling?

If that is a cold air return, it is disconnected. Any open holes on the air handler noted? Look on the side of the indoor air handler. Where is the filter? The return is connected to that side. Has to be a return somewhere. Does the air seem to work OK. How long have you lived here?

We have lived here for 1 year. There is an oblong opening covered with foil tape. It leads to the blower compartment. My aunt has a similar one and that is where it is connected by an oblong flange holding the flex duct. Where should it be connected on the other end and should this be connected?

I can't see your house so I don't know. There should be a square grill, 24 X 24" or so and it should probably connect from the flex to the air return in the handler. You may need to have an HVAC person look at it. If you post some pictures we may be able to help but at present we are just guessing in the dark.

If it is a mobile home there is no return it pulls threw the door of the furnace