I just hooked the +12V flashing light trigger to the left tail light, really easy if you have the keyless unit installed in the trunk.
Just remove the light cover, unplug the 8-pin connectoor, snap off the black cover on the wire plug to expose the pins, find pin #6 (gray/black tail light wire) and solder your wire to the back of it. Bam, all four parking lights flash when you hit the keyless button.
This is for a '91 300SE, so check the wire color to be sure you use the correct tail/park light (might be different color code on yours).

You can also hook a horn relay to this same signal wire, to beep in conjunction with the light flash. But you will need a diode between the Keyless unit/beeper and pin #6 to keep it from beeping when you turn the park lights on. Negative side of diode to pin 6, positive side to keyless unit and beeper.

DG