I soldered one wire to the coil of the relay and another to the 11V supply inside the glow plug controller unit. Then I connected the wires to a toggle switch under the dashboard. So when I turn on the switch it energizes the relay and glow plugs come on.
I had a 77 300D before but never had this problem on that car.
The fast glow/ post glow plug kit is for 5 cylinder engines. It comes with the controller and four glow plugs. Cost is about $95. This kit can be used with an additional relay to energize the sixth glow plug.