Finding the Switch Preglow Time Relay

I have replaced the glowplug sensor, but have the same problem, the glowplug stays activated for 3 to 5 seconds. I the relay on the dash behind the glowplug indicator light? If not how do I find it. I have a 1977 300 D. I have also had difficulty ordering the part through your parts shop. It is part # - 001 545 22 32 I get the message the part is either out of stock or not found on the parts database. Do I have the right part #?, If not can you tell me what is the right part #?, if so can you tell me where I can get the right part? Thanks so much for the help. Charlie

------------------