1986 GMC C1500 1500 Light Problem

I had the dual headlights on my truck, but I attempted to switch from all four headlights to just the two dual-filament lights on the bottom. Now that I've done that, my lights aren't working properly anymore. The left signal light gets stuck on when the other lights are on too; it will only work and blink properly when the headlights are off. The right signal light is working fine, though. Also, when the headlights are off and I signal left, the left tail signal light works properly, but the left license plate marker light lights up and the right tail light marker light lights up dimly. The front-left marker light won't illuminate because of backfeed it seems; both wires leading to the bulb are somehow getting positive current, so there's no negative current. Finally, when you signal left and the cruise control is on, shutting off the left signal also shuts off cruise control. Everything seems to be hooked up properly; I have checked the multi-fuction switch on the side of the steering column as well as the signal light switch in the steering column, and both are working fine. I couldn't find any blown fuses, broken connectors, or frayed wires, so I don't know what else to look for to try to fix this.