I reccently installed an 99 Si gauge cluster in my 96 Civic DX with an automatic transmission. While doing this, I only used 2 of the 3 factory plugs. I didn't think anything of it because the Si cluster works just fine. Also, the remaining plug only goes into the shift indicator lights. It's just that before I swapped the clusters, I would get a frequent CEL because my cat converter is bad. I've had the Si cluster in my car for about 3 months now and I haven't gotten a CEL yet, so I was wondering if the unused plug had some ECU wiring in it? And if it was okay to leave this plug out? Here is a pic of said plug:



Also, I noticed that the auto cluster was missing a plug where there Si had one... right here:

 

the third plug is just for auto indicator b.s and that extra plug is just for cruise control in wich your dx doesnt have it so dont worry about it

 

Is the bulb any good in the Si cluster?