There isn't a COR pin on the UDC as such, but pin E5 there does indicate the status of the mute which may be good enough for your purposes. Its an open collector output, low when muted. E10 is your ground reference pin.
I think you need to set the UDC resistor on pin E8 to activate this, but I'm not certain what value you should use... probably 1540 ohms to set the MRK into data mode. That in turn may depend on having feature 16 activated on your MRK.
E5 pin may also be active when other UDC resistor values are set for particular values eg deluxe vehicle charger, external speaker-microphone etc. I'm not sure on that at all, you'll have to experiment a bit
LBI-38203C has a fair bit of information on UDC pin assignments etc. Just make sure you're looking at the MRK UDC info, as it is different from the earlier models covered in that document. Google LBI-38203C, you'll find it easy enough.