Hey,
First they may or may not be in series. If there is a wire going between each plug then it's in series. But you should have a seperate wire for each plug.
The transition year from loop plugs to pin type was 1980.

The relay is located on the drivers side wheel well. There is a plug that you can pull off with each plug wire numbered on it. You can check each plug with a multimeter. Go from the battery ground to each plug wire checking resistance or OHM's. I can't remember the values your looking for I think anything = or over a value of 1.0 is bad. The Haynes manual has a good writeup on this procedure.
There are also lots of posts on this subject here, doing a search should pull up the info you need.

The relay fuse is in the same black box the plug is in. It's a brass bar that can be checked by removing the cover. Don't just go by looks if you want to be sure remove it and check it that way.

And last, you can buy plugs at AutoZone for about $10 each.

Danny

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1984 300SD Turbo Diesel 150,000 miles

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