Intermittent start issue

[khl]

99 Ford F350 7.3 w/ 148k on it. Bought it used with a starting issue. Sometimes it starts, sometimes it doesnt. When cranking and no start, I here a rapid clicking sound coming from under the dash kind of underneath the radio possibly on the same block the flasher is plugged into but not sure. When this clicks rapidly, thats how I know it wont start because it does it every time. When it doesnt click, it will start every time. Usually I just crank and when it click, I back off on the key and immediatly try to restart and it usually doesnt click anymore and will start. Anyone know of a relay that could be causing this under the dash?

Im in MA, kind of cold 10-20 sometimes 30s. I did new fuel filter, air filter, 10-30 rotella oil, oil filter, howes antigel/conditioner in every fill up. I bought new glow plugs, valve cover gaskets w/ new harness but yet to put them in. Im kind of new to diesels. I plug the truck in every night. I read around the forum and I am changing the glow plug starter relay upgrading it ASAP and I am getting a timer for the plug along with a Marinco plug to plug in from time to time. Just trying to fix this issue as it does it even when the truck is warm. This is a fleet vehicle - main vehicle that carries all of our main tools daily so you can understand how I have to make this vehicle reliable as possible.

Suggestions?