The switch you speak of is actually just a 'half-height' switch (on/off style). Unfortunatly, the low-series BEM which comes in the Forte does not have the antenna control functionality. Ususlly, in mid/high-series systems, the headunit puts out the antenna signal to the BEM (it only gives the antenna up signal when the stereo is turned on AND in radio mode (ie. in CD/tape/radio off, the antenna is down). It is the BEM which then drives the antenna motor. The BEM can put the antenna up to either full or half height, depending on the position of the half-height switch. (Only high-series cars have the switch, but since the mid/high series BEM is the same, you can just buy a switch and plug it straight in if you have a mid-series car). Anyway, this means to get a factory antenna setup, you'd have to get a high-series BEM, but the other problem is the low-series head unit may not have an antenna control signal either, so in other words, the easy way would just be to get the antenna (factory or otherwise) and wire it up to your own two-way up/down switch to control it manually).
Hope that helps.