Ok, so this is a weird weird thing and I don't even know where to start. My cell phone (Evo 4G, if it matters) doesn't charge when I plug into 12v from the truck, and I think it actually drains.

I know it's not the battery, because I swapped the battery for a known good one I had in the garage - both of them read 12.5 volts with the truck off and 14.7 when it's running. I've got 14.4 volts at the 12v socket in the dash with the truck running. I know it's not the 12v USB adapter or USB/miniUSB cord, because I've swapped both of those. If I disconnect the battery from the truck and rig up a 12v socket straight to the battery, it charges. If I plug it into the 12v socket in the dash, it actually loses battery faster than when it's not plugged in. I know it's not the phone, because it charges fine in every truck I've plugged it into except my own.

I'm stumped. I don't have adequate electrical diagnostic skills, so I don't know where to go from here - what could cause this, and how can I isolate it with my little $5 Harbor Freight multimeter?