Depends on the modle year.
Pre 90 cars have 3 plugs on the inside of the car. 3,6,9 pin.
Post 90 cars have 3 plugs behind the batery ( two oval and one square )
All Sierras should have these plugs.
If you look at,
http://www.claytune.org.uk/
Weblinks, Other resources, "Loom plugs ( granada to sierra)".
That gives most of the info on the plugs although it's intended for a slightly different audience.
TT