Alpine Headunit

Hey,
I have a 197 r27 and yesterday i tried removing the headuinit and replacing it with an alpine iva-d106r. I found that there are 3 cables going into the back of the current headunit but having looked at the connections on the alpine there are only 2, with the smaller yellow connection with nowhere to go. I tried plugging it in without the yellow connection but i could not get power to the new headunit.
Could anyone help me with this, as i have had a look round and cant find any information on it!
Thanks,
Chris
 
the yellow plug is only for the screen in the dash, as for power, u couldnt of connected the two ISo connectors correctly