So this is a no-crank problem right, you don’t hear that rrr rrr rrr souind when you turn the key to “start”. And I presume you don’t hear any click sound either. tjhat would be what happens if you forget to push the clutch in, no rrr, and no click.
hmmm … is there a reason you think this is the ECU that’s the problem? B/c I wouldn’t guess that, not as my first guess anyway. What you need to do – or have done – is trace out the start signals that get sent when you turn the key to start. the signal usually goes from the battery to the ignition switch, to the clutch safety switch, often to a small under-dash relay, to the ECU, then to the starter solenoid (the small wire attached to the starter motor). You’ll need a wiring schematic diagram, then have somebody knowledgably trace it out point by point until you discover the first place where the signal isn’t getting to.
You indicate above when you looked at the ECU it was a little damp. Where is the ECU located in your vehicle, inside the engine compartment? If dampness is the cause of this problem most likely it’s just a connector that connects to the ECU that’s a little corroded. I can’t imagine a design where the ECU circuit board could get damp just b/c of rain, unless there was some kind of crack or fault in its covering, etc.
PS, check any fuses that are involved with that circuit, especially if any have that same dampness problem.