Alternator Wire? 2009 Prostar Premium.

I have replaced the Delco Remy alternator with a Bosch. The D/R had hot wire, ground wire and a third wire attached to the post marked Sensor. There were 2 more posts on the D/R alternator labeled R and I, which were unused. The new Bosch alternator has post for 2 Hot battery leads (i used one) a Ground wire bolt, and two other posts, one for tach gauge and one for the alternator light. I have no idea which of these two posts to run my third wire to. I left it unattached (the third mystery wire) to see what wasn't working without wire hooked up. The tachometer worked as well as the voltage gauge, so im not sure which post to put the third wire on. Any help would be appreciated, thank you.