Weird couple questions

[76Blue_Beast]

Ive posted before about my wire harness that Ive had to replace. I have finally replaced this with my little brother. Now we have a problem; there is a really long light brown wire that has a little rubber booty the looks almost like the end of some headphones. We have no idea what it is for. Good news being the truck turns over and is getting fuel (replaced old filter), we figure we would check the coil for spark but cant find where it is located! I feel stupid for asking but my chiltons manual doesnt show it either as Ive read through it several times. We figured checking for spark would be the first thing to do. Starter fluid has already been tried too. Any help. I can get a pic if needed as Im at work right now. 76 gmc c25 3/4 350/350 Sierra Grande

Thanks,
Ken