I'm fairly handy and I cannot find where the filter goes on my Goodman gu 125 furnace from the late 90s. Does anyone know if it is underneath or if for some reason it doesn't have one. Reason I ask is because it is not on the side middle or top or really anywhere on the furnace. It may be in the crawl space underneath the house but trying to stay away from there at. Trying 2 get this switched out because it's been a year so. Thanks any help or even a guide would be great

On those older models the track for the filter may look like part of the housing - and sometimes there's not a track but just a place for the filter to lay.

I'm not finding a schematic for that model; can you attach a pic of what you see when you open the access panel(s)?

You may have a central return filter. Do you have a BIG air return, usually in a central area? Another possible place is right under the burner assembly. I have seen them there on a slide out tray.