The 80 amp fuse that is first inline before the glow plugs keeps blowing, I am going crazy trying to find out why.
None of the plugs or leads or anything seem to be grounding by mistake.
I don't know what to do.
Any suggestions much appreciated. I am trying to sell the car and clearly can'[t with this problem.

1980 300TD Wagon, non turbo.

I don't know but it would seem that a short to ground after the fuse would have to be the problem.

Get a couple of fuses, disconnect all the glow plug leads/wires put in a new fuse.
If the fuse blows with no GPs or their wires connected that would point an internal relay problem.
If the fuse doesn't blow one by one reconnect each glow plug until the fuse blows again! I could be more than one short so you would probably continue to check all the GPs in this process of elimination.

You could disconnect the wiring between the relay/fuse end and each of the glow plugs and check each wire for a connection to the engine ground. You could also check each glow plug similarly for an internal short to ground.

There could be an internal short to ground somewhere in the relay circuitry itself. This is assuming that the fuse is in series with the relay mechanism and comes before it in the circuit. The simplest procedure would be to swap the relay out with a known good unit depending on the cost and availability it might be most economical to hit the bone yard and grab a couple from a similar car for swap/testing. New parts (relay) looks about $75 -100, Fastlane lists an upgrade package with four new plugs and a new relay with a revised 3 min. after glow period they also list a glow plug insulator so perhaps there is a possibility that an insulator could fail or break down. I don't know what this insulator looks like or where exactly it goes. Good Luck!