W123 300d Glow plug issues

[Bmore300d]

I have been searching all over and apologize if this has been covered. Below is the issue I am running into with my pre-glow system. It weird and I can't seem to locate anything related to this any where and this to comes and goes, sometimes the system works perfect other times it does not, about 5% of the time.

When I insert the key and turn into the glow position, the light comes on...95% of the time. Sometimes, the light flickers or does not come on. The weird part is when the light flickers, I hear a strange noise coming form the firewall area under the speaker that sound almost like a clicking noise but is more of a deep stepping buzz behind the seat belt relay box.

Initially, I thought to change the plugs, so I did all 5 to eliminate those. Checked the plug relay with a meter, all seemed fine, as well as replaced the strip fuse. Now I am thinking the glow plug relay could be bad or maybe the ignition switch might be failing. Wanted to check if anyone here had the same experience with their w123 and could maybe offer some advice about what I may be missing in this equation.