re: Need "C"Wire (5th wire) for Wifi thermostat.

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You only need an add-a-wire kit if you do not have enough wires. If you have at least one wire not hooked up at the unit and one at the thermostat you can have a C wire. 


This is AC power, but lets pretend it is DC.....Red wire is positive and C is negative. All your thermostat does is switch the positive to the other wires you hookup. On the board at the ac those functions already have the ground. 
What these new thermostats need is positive and negative to power the electronics in them. The wires running to the old thermostats were just the positive, the C provides the negative. 

In AC power terms, you have a hot wire running to the thermostat (usually the red), but you have no neutral in the old setups. The C adds the neutral. If you have an extra wire in the bundle, all you have to do is connect it to the neutral side of the transformer (sometimes labeled as C or Common). 

In the picture from the OP, any of the wires labeled C would work. Looks like you have a blue wire not being used between the unit and thermostat. All you need to do is connect the blue wire to one of the C (brown in the picture) and you now have C at your thermostat. 

Side note, make your connections at the thermostat with the unit unplugged or the fuse pulled. If you accidentally touch the C to the red, you will blow the fuse or possibly the board. 

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