I'm looking to upgrade my old thermostat and need to run a c-wire. Where would be the best place to tap into?
Where would be the best place to tap into?
whatever side of the transformer goes to Rh or Rc on the thermostat, the C (common) is the other side of the transformer. From that other side of the transformer or tapping off the wire from that other side of transformer goes to the C terminal of your new thermostat.
You would want to tap into the blue wire coming off the secondary side of the 24v transformer. You can see the transformer in your first picture, it has the white and black wires going into the top and the blue and red wires going into the bottom. You want to tap into the blue wire. Now you may have noticed the blue wire also goes to ground... Do not take a short cut and attach your c wire to ground. You can't use ground to carry load, so splice into the blue wire. good luck.
Connect it to the junction of the fat blue wire and the skinny white wire
We know the fat blue wire in the foreground is a C wire, as it's currently returning current from the air conditioner compressor contactor (via chassis ground, which is odd, but it's what the wiring diagram on this unit calls out) already. The easiest place to splice into it is at the existing wirenut junction, although it appears you may have to hunt around for a spare wire in your existing cable if one is present but not visible, or pull a new thermostat cable if your existing one has no spare wires in it.