There shouldn't be any relays on that vehicle to stick so it's got to be the switch that's all that's left. That was/is a common problem on all Ford vehicles of the late 1980's and early 90's. I remember that there was a big scuffle about the headlight switch shorting out and catching on fire but can't remember what years it involved. I know it was on F-150's but not sure about the E-150s though they should have used the same switch. I don't remember all the years it covered and it could include the 92's . Check with Ford on that. But it's probably the headlight switch anyway. I guess the original switches were made in Mexico and went bad around 70K miles or so with some causing fires so please be careful. Supposedly the headlight underdash wiring looms were melting and catching on fire also as they were made in Mexico too. Wouldn't hurt to keep a charged up fire extinguisher in the van just incase from now on after you replace the switch. Sorry I couldn't be more help. and Good Luck,
Kenny / MrBlueOval