Diagnosing preglow timer relay electrical problem

[w2pa]

1987 300DT

I recently replaced the 80 amp ribbon fuse on my preglow timer relay. About 3 days later, the new fuse failed.

The glow plugs and starter are less than a year old, so my guess is that the problem is NOT there.

I'm an IT guy by trade, so circuit testing and troubleshooting is not a foreign concept (though I usually work in milliamps, and this has a bit more juice to it). Can anyone point me to a decent primer on how to isolate what's melting this fuse down? I'll even take the simple answer, but I would love to sleuth this one out, as the DIY was one of the reasons I wanted one of these older 124's.

Thanks in advance!