There is a common (aka "ground") wire from the back of the engine to the firewall you should check/clean.
I've found that can "look" fine, but the firewall connections has corrosion built up. Check that the connector is good, clean both the connector and firewall mounting, and reassemble (I like to spray some silicone after assembly to help "weather proof" the spot).
BTW, this is a very important connection!!!