Here is an update to my own thread:
1. I accidentally found a new OEM replacement trailer wiring harness. It fits an unbelieveably wide range of vehicles, but is supposedly plug 'n play for quite a range of Mercedes, including the 1999 ML 430.
http://www.***************/store/?N=11221+4294963846+1668+11921+5641.
2. Could this possibly be a quality replacement part for the extinct OEM MBZ trailer wiring harness??
3. My search of dealers and countless wrecking yards failed to turn up an original new or used trailer wiring harness.
4. I removed the left taillight assembly and tested the wires with a volt/ohm meter. That gave me the answer to my own question about how to hook up the wires. Hookup would be a no brainer, but a good quality plug 'n play would be better.
4. Any comments??