Turn your shay upside down. Centered between the journal boxes/springs on each side of each truck are two screws (one on each side) that point towards the water tank and boiler. Remove these screws and the truck comes loose from two posts and the pivot point that house the contact rings. The wires are fed through there. I’m not sure of color combination as my shays were both different, but it is fairly self explanitory after you drop the truck off the frame. You should have four wires though, track power in and truck power out. Figure out your combination of wires and solder together.
There are two screws that are right below the ones you removed they point in towards the center of the truck. If I remember correctly, (I run battery power so I completely removed my track pick ups and wire) these hold the contact strips for track pickup, if you accidently remove them or need to remove them to get clearence, you might want to open the bottom plate of the truck to make sure you resecure them properly inside.
Hope this helps.