Relay mod is a work around, not a fix. Look for jetjock's post(s) in that thread and the read the last page.
If it's firing up while in "N" but not "P", try wiggling the shifter a little bit while in "P" and trying to crank at the same time. See if it fires at all. The switch might not be tripping in the "P" position properly. I had an old beater Mazda 323 that had that problem once in a while, but I just smaked it a bit and lived with it because it was a $450 beater.
I had a similar problem with intermettant no starts that eventually (after a few months) turned to ALWAYS no start and it ended up being a bad starter. The relay mod in that case would have done absolutely nothing. I'd say to start by checking your grounds, making sure they're good. Check voltage drops between the battery and the starter as well as the battery and the starter solenoid.
I'm not saying the starter mod won't make your car start, it just won't fix the existing problem. If you feel like cutting in to your harness that's your own decision, just remember that ever new connection point you make is another point of possible failure.
