Let's clarify. All of the pins except Ground should show 12 VDC when the associated switch in the tow vehicle is turned on. In other words, the pin for the backup lights should show power if the ignition is on and you put the transmission in reverse. The pins for the left and right brake / directional lights should have power if you press the brake pedal. The running (marker) lights should have power if you turn on the headlights. The brake pin should be energized if the ignition is on and someone is stepping on the brake - however, since the vehicle is not moving, the reading on this one may be wonky, depending on your brake controller. And of course the 12 VDC battery charge line should show power as long as the ignition is on (many cars) or all the time (other cars).
If none of these functions are activated (no one is stepping on the brake, for example), then the only pin that should show 12 VDC power is (maybe) the battery charge line. Is this what you are seeing?
As for using an ohmmeter on the trailer half of the the connector? You should check only the lines to the brakes and the lights. In each case, you should expect a reading of a few ohms, say in the range 2 to 10 ohms. If you try to measure the 12 VDC battery line with an ohmmeter, you will get a meaningless reading at best, and destroy your multimeter at worst. I don't think you risk blowing any TM fuses, though.
Tim's question - did the lights ever work with this vehicle? - is a good one. The answer will steer the troubleshooting steps.
Bill
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