Trickle power is a low level power supply that is regularly working on the Mac Pro when the power is turned off. It is supposed to supply VRAM and charge the RAM battery.
According to your checks you said that you do not have trickle power any more. This symptoms are common if the fuse in the PSU is blown. The fuse is a small inexpensive bulb with a little wire inside. It is meant to blow when the PSU is exposed to spikes from lighting and the like. Unfortunately the Fuse is not a standard exchange item for Apple. So you pay 300$ for 1$ part if they exchange it for you. Based on that consideration I thought it is worthwhile to check if the wire is burned through. You can check that visibly when you have opened the PSU.
