'65 Chevy taillamp wiring issue

[nxtruck]

Guys,
My son's '65 has a wiring issue that doesn't make any sense to me. The problem we're having goes like this: when you pull the headlamp switch out to the 1st detent, the ft. park lamps and the rear running lamps come on like they should. When you pull the headlamp switch out to the 2nd detent, the headlamps come on like they should, but the rear running lamps go out. At first I thought I had a bad h/lamp switch, so I replaced it, but that didn't solve the problem. The PO has a toggle switch wired up and hanging below the dash to turn the rear lamps on, but I'm not sure what he could have screwed up with the factory wiring. According to the shop manual, there is only one wire feeding the rear running lamps, and they do work off of the switch when in park lamp mode. Is there anything obvious that I'm overlooking here?
Thanks in advance!