I don't know if the Econoline chassis is the same as the F-series wiring, but I'd check first the fuses, they have multiple ones, one is a 'chassis lighting' circuit, one is the 'trailer lighting' circuit.
If someone plugged in a trailer, or the trailer wiring got pinched, and there was a short circuit the fuse would pop.
If the front light is flashing the switch is working.
If you are getting power through the running light circuit jumping into the right turn/brake circuit it tells me you have some corrosion or something causing a partial (low conductivity, or high resistance) short circuit.
If you live in an area where winter conditions cause corrosion I'd start with the trailer wiring, especially if there's a trailer wiring plug.
Ted 'n' Laurie, plus Jackson (aka Deputy Dog, the Parson Russell Terrier 'fur kid') and, Rylie (who crossed the Rainbow Bridge June 14, 2012).