thermostat c wire question
You should be able to see onto the board where the rest of the wires connect and find the "C" terminal. It likely already has something connected going to one of the many wire nuts (and yeah, that is pretty sloppy). 

You can test for sure using a multimeter. Set it to measure AC voltage, and then between the Rc (or Rh) and C wire you will see around 24VAC. 

Once you've traced C and verified it, connect it up to your blue wire. Do yourself (and anyone who works on this in the future) a huge favour and label each wire at the thermostat before you put it back in the wall.