*One more mkiii that won't crank =(

[El Supracabras]

I agree, throwing parts at it isn't an efficient way to go about anything. First you figure out what is wrong before replacing anything at all. Its called diagnostics. If it had been done, you would likely only have made one repair.
Something is not completing a circuit here. A simple test probe light can pretty much diagnose this, buy one or borrow one. Any schematic, diagram or manual is available on the internet for free somewhere, you just have to look hard enough. Try to use the actual tsrm info over hayes or chiltons. They are helpful but seem quite generic in their info and diagnostic steps.
With the correct starting system schematic, a test light and a simple understanding of how to use both, you should find your problem quickly. Good luck.