should you use only new ford parts or can you use after market parts?

OEM parts are usually specifically designed for the vehicle they are intended to be used on, aftermarket parts are made with a broader market than that. The same part from the aftermarket may fit other vehicles as well. This does result in some comprimises, but most quality aftermarket parts will fit and work just fine. There are lower quality parts out there that can create problems, however. It should be noted, that because of the age of your vehicle, you may have some trouble finding some parts at your dealer. Some parts that aren't used on the newer models will be obsolete after about 10 years or so. The parts department wants to stock items for the cars they service the most, and that would be newer vehicles than yours. This isn't always the case, however, if there is a good demand for a certain item. If the part is obsolete, than the aftermarket is your only alternative.