Found this in a Starlet workshop manual, looks like disconnecting the plugs from the air bags and seat belt tensioners is what causes the problem. The plug has a device inside it which shorts the 2 connections together and shows the item is missing, I think anything you do will not work unless you cut the unplugged plugs off. If you still have your old air bag, I would put a meter across the 2 connections at the steering wheel airbag connections, see what resistance is there, if none, cut the plug off the loom, tape up the 2 wires seperately and see what happens. I would disconnect the battery and leave it for 5 minutes before doing any cutting. If there is a resistance accross the air bag connections, cut the plug off and put the same size resistor on there. The only other option to cutting the plug off is plugging the air bag plug into it and doing the connections the other side, but that renders the old air bag useless.
I dont have any airbags in my car so I cant do this myself and tell you exactly what needs done, sorry. But its a good one to work on. There may be the possibility of working out how every EP could be sorted if something could be made to plug into the airbag loom plug when removing steering wheels so the system still works.