Driver side defroster not blowing

[msbmt]

Hey everyone, I've a got a 78 Blazer that I repaired the blower, heater core, etc...(the last guy butchered a bunch of stuff...more than I thought when I bought it) The vents and heat blow great, but when I put it on defrost only the passenger side blows. Any ideas on why the drivers side doesn't work. I've looked for ducting diagrams and anything of the sort and can't seem to find any.

Thanks