If we are talking about a furnace here they usually have from 2 to 4 panels on the front that lift up and pull off; sometimes they have screws in them. With the bottom panel off your fan assembly should slide out on tracks. I would not cut the wall until I could see for sure the access was on that side.
If you would include brand, model, serial and maybe some pictues then Jeff1 could solve your problem. The guy is amazing.