I just replaced the windshield in my 69 with one that has the antenna built in ($125 from Safelite). It's a '70 windshield, which will fit 68 thru 72. There is a small (about 6") pigtail on the lower edge in the center that is the antenna connection. The connection is made to the cable shown by mr409. If the 68 is the same as 69, you should have square cutout in the sheetmetal just below the center of the windshield, mine had two dimples in it that I drilled out to install the antenna cable. After the windshield is installed, the pigtail is plugged into the cable and then covered up by the stainless steel lower windshield trim piece. Very clean install, looks factory. Personally, I hate the fender antenna, breaks up the clean lines and (stupid) people are tempted to break it off. Here in the Denver area, the windshield antenna performance is adequate; They are less efficient than a standard fender mounted antenna, so if you are in an area with weak signal strength you may not get all stations.
Good luck with it!