Get a voltmeter and a long automobile wire and test all wires that come from starter to different parts of car. Also check all fuses and fusible links. On some cars there is a second box with fuses under the hood close to battery normally. There are bunch of fusible links right next to starter and two near battery compartment. Find how your egnition wire runs on electrical scheme and test it all the way to egnition switch. Also you can try to simply disconnect battery for about 5 minutes, it will reset your computer. Other than that it is a pain on the neck to fix it. Also it will be very usefull to go to dealership, electical shops, walk in through the back and discribe the problem directly to mechanics. (think of a good excuse for them to help you before you go)