Dandy. That means there's a break in the gray wire or a bad ground. Don't overlook HMAC300's comment about mice.
I don't know if this applies to your vehicle, but in the '80s and '90s, GM had a lot of trouble with bad grounds on very rusty vehicles with metal gas tanks. The ground was made through the straps, not a wire to the body or frame. If you have a plastic tank, don't worry about that. Your ground wire will go to the body. If you have a metal tank, move the ground clip for your test light to a paint and rust-free point on the body sheet metal or frame. Do the two-second test again. If you find 12 volts now, there's a break in the ground circuit. You can verify that by probing the tank with your test light. (Ground clip is still on the body or frame). If the ground is broken due to rust, the test light will light up but a little less than full brightness during the two-second test and during cranking. Any voltage on the tank proves there's less than a perfect ground circuit.
If you still don't have 12 volts, there's a break in the gray wire. I've heard many stories where GM runs wires to the rear under the carpet on or near the transmission hump in the floor, and a lot of people find wires rubbed through there.
A lot of people want to run a new wire to replace the broken one, but I never allowed my students to do that. Reason is you don't know where or why it broke, so you can't make an informed decision as to whether another problem is going to occur soon with one of the other wires. This is true of a mouse nest too.
If the original wire is not cut off on both ends, where it's broken can rub through and short to ground. If the wire broke because it's under your feet and has been rubbing back and forth, or it got cut from laying on the sharp edge of a metal bracket, how long will it take before the next wire does the same thing? You need to find the location of the break so you can insure the same thing doesn't happen again.
Saturday, January 2nd, 2016 AT 4:17 PM