To be certain, you should:
* Consult your owner's manual: This is the best resource for the precise location for your specific vehicle.
* Use a repair manual specific to your 1999 GMC Jimmy: A Haynes or Chilton manual will have diagrams and photos showing the location of all components, including the air pump.
* Look online for pictures: Search for "1999 GMC Jimmy [engine size] air pump location" on Google Images. This can help visually identify it.
Keep in mind that some 1999 GMC Jimmys may not even *have* an air pump, as their inclusion depended on emission standards and the specific engine. If you can't locate it, it's possible yours was removed or never installed.