To find it, you'll likely need to:
* Look under the dash: Feel around for a small, usually square or rectangular, box with a button on it. It might be attached to the side of the dashboard or tucked away in a less visible area.
* Consult your owner's manual: If you have the original owner's manual, it should have a diagram showing the location of the inertia switch.
* Search online: A search for "1985 GMC Suburban inertia switch location" might yield pictures or forum posts from other owners who have located it in their vehicles.
Remember to be careful when working under the dashboard; avoid disconnecting anything unless you're sure what it is.