I would start by testing continuity all the way from the backbox to the playfield lights and back the other way. Don't forget to test across the GI relay as it's possible for them to go bad. Also, check for voltage - AC, not DC - in the playfield GI circuit.

There are two relays, one for the backbox GI and one for playfield. The playfield GI relay (if I recall correctly) is on a small circuit board (about 2" x 2") under the playfield near the back.

GI bulbs run on 6.3V AC. You could try pulling the GI connector in the backbox - I believe it'll be labeled on the bottom circuit board - and testing the header pins with your meter.