what is the light blue plug on the back of the cluster for i went to swap out my std cluster for a si one and the si did not have a place to plug in the blue plug and the bracket holes did'nt line up right. could the si cluster be out of 88-89 ef cause they say 88-91 differ from 90-91.

 

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I don't know what the blue plug is for, some have it , some don't- even same year models. You can tell what year cluster by the shape. 88-89 the top of the cluster is flat while the90-91 cluster is more rounded. There are probably differences in the 2 plug pinouts. I have read that some have used the cluster withuot plugging up the blue plug and had no problem, I hafe never tried it.
Dan