While attempting to rewire a duplex receptacle under my sink to power a garbage disposal and a dishwasher (half-switched), I ran into a problem. The existing receptacle only had 12-2 romex and therefore lacked the two separate hot wires necessary. Just as I was about to abandon all hope, I noticed a small hole in the paneling near the receptacle I was working on. Curious, I decided to open it up with my drill and I found a blue box with 5 black, 5 white, 5 bare wires tied off (photo below). Is it possible use these wires to install a gcfi outlet? If so, what's the best way to go about it? I should mention the home was built in 2011 and the kitchen outlets are all gcfi. Thank you for any advice you may offer. Apologies if this is a double post.
If you can't use the interesting bundle of wires you found, there might be another approach. Assuming the 12-2 romex came from the disposal switch (probably above the counter) you might bypass the existing switch and make the current wire to the disposal unswitched. Then you can add a switch and receptacle in the cabinet (assuming it can be made easily accessible under the sink) for the disposal and a second one for the dishwasher.
btw - my house was built in 2004 and we had to install a switch for the dishwasher as well to cut power for maintenance. If you dishwasher plugs in a way it can be unplugged for servicing, then you probably don't need a switch. But if it connects directly to the circuit, you may also need a cutoff switch.
Also you would need to confirm the circuit has enough amps to service a disposal, dishwasher, and whatever else may be on it.