The fix for this is really straight forward but it is time consuming. As for where to start, well start at the start. Get a copy of the wiring diagram and just follow the path from the battery to the coil OR work from the coil back to the battery.

I personally like to start from the fuse box......make sure power goes in and comes out.............I check the connections on the backside of the box, I've had at least 3 old cars where the solder connection on the back was bad.

I just use a test light........sometimes I find the electrical fault in 10 minutes and others it takes 3 hours. If you only have time to spend 20 minutes on it at a time that is ok too.........just write down were you left off.

Also what Dave said about new cars........my 2001 Protege is been having fun at my expense, appears to be main relay. Turn the key and no CEL light, click the key on and off......wait and you get a CEL light......letting you know the ECM is working. Tapping the relay seems to fix it of course it has not done it for 2 weeks. I have a new relay just waiting for it to happen again.

Tom