Installing an Ecobee3 - Add C Wire with PEK
Alright -- the trick with the PEK for you is to treat everything wirenutted to your two thermostat cables as the "furnace" from the PEK's standpoint.  You'll need some 18AWG pigtails for the connection from the PEK to the "furnace", though, as follows:


Red/black nut to PEK cover (furnace) R
White nut to PEK cover (furnace) W
Green nut to PEK cover (furnace) G
Yellow/white nut to PEK cover (furnace) Y
Blue/red/white nut to PEK cover (furnace) C


and then you can connect the base as follows:


Red Left to PEK base (t-stat) R
White Left to PEK base (t-stat) W
Red Right to PEK base (t-stat) G
White Right to PEK base (t-stat) Y


Just make sure to connect the Red Left (R) wire at the thermostat end to the Ecobee's Rc terminal or else the Ecobee won't turn on (this is a limitation of using the Ecobee/PEK combo).