Random misfire simply means a non specific misfure, ie summat not quite right but not enough to trigger an individual cylinder code... Is the wire still attached to the ballast resistor?
It lives behind and below the rear multiram valve, bolted to the 246 head.
The wire is brown, but most definitely
NOT an earth... damhik

but water from the scuttle corrodes it at the resistor, causing it to fail.
Solution... new resistor from VX,(2.6 Vectra B ones readily available), take the cable tray cover off and follow the wire into the loom. You'll find it crimped to the main power feed from the ecu to the coil packs... cut it 2" from the crimp, and solder 8" of new wire to it (sealing with either tape or heat shrink tube). Rebuild the plug using tother end and plug into new resistor.
Don't look for the resistor or wire when the ignition is on or the engine is running, as it connects directly to the coilpack feed...