Turn signal wiring/switch theory confusion

[jds8086]

Working on a '71 C20, the owner (a friend) replaced about every wire in this truck and most everything seems to be working, but we are having troubles figuring out the front side markers. The diagram we have shows that one wire goes to the running lights (brown) and the other to the turn signal wire for whichever side it is on (LT blue / blue). Things aren't working as they should and all i'm really after is for someone to explain how this setup is supposed to operate. For instance if you had your parking lights on, and turned on the turn signal, how would the side marker flash if it has constant 12v on one wire (parking light, brown wire) and alternating 12/open on the turn signal wire? If i understood how it worked i could probably get it figured out but it just isn't making sense to me. Any help is appreciated.