Quoted from Slim64:I believe there is a relay under the playfield that controls the GI. If you have a dmm, check both GI lines for voltage. That will tell you what to check next
Tracing wires is easier when you rely on the colors which usually follows consistently. I would look in the back box to confirm that you have power off of the power supply.
if you have power to the line side of the relay 5v(yellow wire?), you should have something on the other (load a or load b contacts) bad relays freeze open or closed. Don't confuse it with the coil side 25v (brown/black, brown/red?) is all of your GI out? both white and green? how about back box GI? if you don't have power at the relay make sure you have it coming off of the power supply. check both sides of the fuse in the back box, right side second fuse from the back, yellow/white wire
Looking at the schematics, it appears that the GI ckts come off of the power supply board connector 3J8 (pins pins 7 and 2). That connector is notorious for burnning up. inspect it close for any discoloration. Heni makes a good suggestion of buying a molex connector kit. the question is whether to buy the IDC or the crimp style. I prefer crimp style, but I'll sell you my IDC's if you wish(I bought a set specific for this game). There is lots of molex discussion, read more here http://www.pinrepair.com/connect/
It looks like solenoids 9, 10, 11 control GI, Q9, Q16 and Q17. you might test those transistors.