OK so I ponied up for the actual Volvo wiring harness for my trailer. It terminates in a flat-four connector (standard for light trailers) and does the "4-bulb to 3-bulb conversion" (i.e. the brake lights and turn signals are on the same filament). And it worked great for a couple of months even including lighting up the auxiliary turn signal light on the lower right side of the dashboard. But now it doesn't work at all. It's not a problem with the trailer, I tested it with a tester also. Any suggestions? How about a pinout for the connector near the spare tire so I can see whether the problem is the car not sending signals to the adapter or the adapter not passing them through to the external connector...

Thanks!