Meter in hand, set to AC. Don't measure AC to ground measure hot to common. For a start measure AC onto and off of the power driver board and see if GI power is going in or out. That will help you narrow down where to go next.

This is assuming you pulled GI fuses and checked with a meter ?

LTG

Any Burning or discoloration on plug or header pins at board?

I had a similar problem dannunz when I picked up my Shadow...
A strand was out and a toasty J120 was the culprit.
The GI wires for the upper and lower BB are on J120.
You should check wires first to make sure there is a good connection to the IDC, re-seat them if they seem loose and see if that fixes it. (Easy stuff first... Right?)
Pulling the connector and inspecting it will show you if it's burnt. If so, I would re-pin J120 (hopefully with the higher temp black header version) and redo the connector/contacts too. You can get all the .156 parts from here...
http://www.greatplainselectronics.com/Category-37.asp
Since you didn't really specify which portion of GI was out, I'm guessing it has the same issue.
Here is some "before and after" pics of my job. (and don't worry, no flash was used for said "pics".)

Good luck!

Toasty! (pin and junky orange connector)
IMG_0718.jpg
New header, connector and freshly crimped contacts.
IMG_1425.JPG
All better now...
IMG_0732.jpg

Definately check 121 and 120, pull them off and look check for charring

I'm too tired to think. But you may as well keep eliminating things on your way to fix things. Fuses are always a good start.

LTG

Def check J120/121. They are the reason I like LEd's in my backboxes.

No. There are 5 GI fuses. Two are for the backbox and three for the playfield GI.

The wires are color coded. Check connectors j120 and j121 for the color stand that's out.

...

Fuse pulled and checked with a meter ?

Then I'd pull the board and with your meter check continuity from the pin, onto the board away from where the pin is soldered on. That will tell you if you lost through or across board continuity.

LTG

In the words of the almighty Chris Rock...
"What am I, GLASS?!?"

Your meter has two probes, stick one on a header pin on the power driver board. Follow the trace away from the area and put the other lead on a bare spot or solder joint away from the header.

If you have continuity move on to the next pin. If not, start running jumpers because you lost through or across board continuity.

If you need to understand continuity - stick your two meter probes together with meter set to ohms and note what the meter says or reads. Then when you check from a pin to another spot on the board, you should see the same reading.

LTG