Trying to figure out 'C' wire
I'm not sure exactly what's going on with the wiring, since in the one photo there's a giant ball of wires and cables, and in another there's extra wires hooked up. Here's what I see in the photos you've posted.



In this photo, you'll notice the cable with a red and white wire. That cable should go between the furnace and the A/C unit. The white wire from the cable is connected to Y, while the red wire is connected to C.

The other cable I've labeled should go to the thermostat. The wires from that cable should be connected as follows:


White to W
Green to G
Yellow to Y
Red to R


It's difficult to tell if that's what's going on, due to the big mess of wires and cables in the middle of the photo.

Also take note of the blue and black wires, that are wrapped around the thermostat cable.





In this photo, the cable coming from the furnace looks completely normal. However, there seems to be an extra cable that comes from who-knows-where. It appears that the wires from this mysterious cable, are connected to the blue and black wires that at the furnace are wrapped around the thermostat cable.  Which if that's the case, they're doing nothing.

It's possible that you might be able to attach the blue wire that is wrapped around the thermostat cable, to the C terminal at the furnace. Then attach the blue wire at the thermostat, to the C terminal on the thermostat. Though without knowing what the mysterious cable at the thermostat is, there's no way to be sure.

I wasn't able to decipher the diagram on the wall behind the thermostat, so I'm not sure what that's all about. I'm sure it meant something to whoever drew it, but it's not helpful to me.

I'm not sure if you have multiple A/C units, and the extra cable is used to control that. You'll have to investigate further, to determine what exactly is going on with that.