Goodman furnace approx 20 yrs old! Won't come on to heat! Its like its not getting command to heat! Replaced thermostat! Same thing! One time I recycled power from circuit board and it fired up while I was holding in safety switch! Once I let go and put cover back on same thing! Any thoughts?? Thanks

If you hold the switch and it heats, looks like the element is good.
The thermostat usually connects the red(24 volts)to the w or white wire/terminal. I would jump at thermostat for a test or verify switching of thermostat contacts.
I wonder if the safety switch is to disable the unit if the cover is removed. Check any switches and look for a diagram. 20 years ago the boards weren't too complicated.