Do you have the small relay box (about 2"x5") that attaches to the radiator of the ST205?
Can you post pictures of where your harness is cut?
You should be using your car's fuse box anyway, all you have to figure out is the wires that you need to connect for power/ground (all the fuses on the ST205 match up to the SV21 in name and rating). You can keep all your factory chassis related wiring (charging, backup switch, etc.) and just change plugs as necessary. Use all the engine specific stuff from the ST205 and pull our all the 3s-fe specific stuff from your factory harness. After that, you just put them together (extending/moving wires as necessary to fit the chassis). Take good notes, lots of pictures and make sure you label every wire you cut.
If you have more specific questions, I can help out more.
-Charlie
PS. For others reading, this is why you buy a complete clip instead of an engine set. You don't save money by getting an engine set - you can sell parts off the clip to recover the extra initial cost after you are done using what you need).