Yes you do have to run the wire to the connector near the ecm....some looms had this wire already run so check this out ..its a yellow/black wire.
You can solder the wires off the old loom but it is a bit fiddly....and you have to undo the loom near the fuses so you have enough wire to tee off.
Here are a few diagrams of the wiring connectors that might help
You need a connector and wires that feed the mode/reset switch that goes in place of your speed alert but if you are stripping back the necessary wires from a short loom they will still be connected at either ends in the connector boxes anyway.
If you are going to do it without pulling the dash apart you will need to run these wires behind the dash .try
http://forums.justcommodores.com.au/vr-vs-holden-commodore-1993-1997/38452-vs-dash-upgrade.html
In diagrams first one tees off wire at connector 124,125 and 126 end and the next shows it at instrument connector 1 2 3 end...same wire just different ends.