I have an RS2000 mk5 track car thats converted to run a Zetec coil pack.

The RS orginally had 2 packs using 4 wire, but 1 of those was split into 2 and ran to each coil pack, this is the common earth/return for each coil. When using the 'single' zetec coil pack you only need 3 wires, so just turn that one back into a single and it will work just fine.

On the zetec coil pack the middle terminal/connection is the earth/common return btw