You should be able to find it at Deere.com or I picked one up at the local Deere dealer when I needed it to fix my friends 4020 light. I don't know where they are now, or I would scan and send.
If it were me I would rewire the lighting circuits on a machine with that much age. The wiring wasn't protected that well and new sleeving and wire would eliminate alot of future problems with it. At the least I would go over the wiring with a fine tooth comb looking for abrasions and cracks. This is a low cost problem solver imo.