Easiest way, IMO to do this now is to use a relay to interrupt the circuit that signals the Legend to flash TD/AL (BLK/RED...or maybe RED/BLK, can't remember). To wire this in, you will have to control the relay with the park signal circuit from the vehicle. Sometimes this is ground, sometimes its positive, sometimes it is polarity changing. You will need to know which and how to safely interface with it. Basically, the park signal from the car will control the relay switch by either powering or grounding it. You would the wire the relay as follows...
85 - park kill signal
86 - opposite potential of above
30 - power input from lighting mode 3 output
87a - TD/AL flash wire from Legend Serial Interface cable
87 - No connection
You can also use a Fed Sig/Unitrol UPKM module, which really is a relay set-up as well.