the glow plug system on a 1994 IDI has a controller/relay in one. you can replace the whole thing. you can replace just the relay but usually if one is bad the other is too. the controller piece tells the plugs how long to stay on.
List: Search for 'glow plug controller' | O'Reilly Auto Parts
then you have the plugs themselves. you need to make sure they are motorcraft ZD9's
Motorcraft ZD9 - Ignition Glow Plug | O'Reilly Auto Parts

the only other thing could be power delivery issues which even a manual switch couldn't prevent that you could end up frying plugs..... i suggest getting the stock system in working order.... i love just being able to click the key forward and having no extra buttons to push but thats just me