I would check with other Forester owners at nasioc.com, subaruforester.org, or your local Subaru forum to see if anyone has gotten a stock stereo from an '01+ Forester to fit in a '99. If so, then the bolt-in should work fine.
Worst case, however, you'll need to drill new holes in the mounting brackets in your car to mount the TS kit to.
Or, you could pull your current stereo, take out a ruler, and compare the mounting screw locations you're using on your current setup to the schematics for the TS kit here: http://store.mp3car.com/ProductDetai...Show=TechSpecs
Good luck and be sure to let us know how the install goes!
Oh yes, and welcome to the forum!