mystery in the rear wiring... got me puzzled

[bake74]

     74 k10 step side.  My issues are with the rear lights only.  All front lights work the way they are supposed to.  So first the issues and then what I have done so far.  My truck sat for a couple of months without driving it, just started on the weekends.  Went to jump in and take it to work 2 days ago and no lights on the rear except the license plate light. (all lights worked perfect before that morning)  
     Today I checked to see what was going on, here is what I found.  Drivers and Passenger blinkers will activate both side markers and both filaments (dimly) in both bulbs in the rear lights.  I have no running lights at all in the rear.  Emergency flashers will activate all 4 lights in the back (side markers and rear).  I have no reverse lights.  It seems that my brake lights work (only the high filament in the rear bulbs activate) , but the side markers come on with the brake lights also.  NOTE: all the front lights work the way they are suppose to with their proper functions.
     Now what I have done.  I replace the wiring harness from the connector in the engine bay all the way back to the rear with a new one.  I have checked all my grounds on the whole truck but especially the rear side markers and rear lights assy's.  Checked and doubled checked all bulbs in the rear of the truck.  checked the ohms on the rubber connectors for the rear lights on the step side, they are ok and not shorting out there.  I have checked several spots on the rear of the truck for proper grounds and have great ground on all the spots needed.  I also took the 4 prong connector apart in the engine bay and checked each wire going to the rear of the truck for dc voltage to ground with each corresponding function. (blinkers, brake, running lights, reverse, hazard) and they all checked out(no other voltage on any other wire except what I was checking), the only thing I am not sure about is with the blinkers activated(either side), the brown wire(running lights), 18 brn-9, has fluctuating dc voltage on it, goes from .1 to .2 to .1 to .2 etc. etc. at the 4 prong connector.  I even took off the main junction block on the fire wall and checked with an ohm meter.  I also checked the new wiring harness after installation at the rear to make sure I have power on the right wires corresponding to proper functions.
     Sorry that this is so long, just wanted to cover my basis.  So if any body has any idea what I need to check next, I am game.  I am out of ideas for now, and thinking it is something causing a cross somehow that only affects the rear lighting, but at this moment I am clueless.  Thanks in advance for any help.  

[1979C20]

My buddy had the strangest problem with his lights in his 76 c20. With this parking/headlight switch off, the brake lights didnt work, the left blinker didnt work, the right blinker did. All bulbs good, wiring good, fuse good. Parking/headlight switch on, the running lights worked, the left blinker didnt, right didnt, brakes didnt. Checked all the wiring for breaks and checked all grounds. Checked brake light switch. Turned out, there was a 4?wire plug under the dash for the blinkers(i do believe) that had come loose and wasnt completing the circuits. Look under your dash above your pedals and along the fire wall for loose plugs!

[bake74]

     Thanks 1979c20, I checked and there really is not any plugs under the dash anywhere that is connected to the rear lighting.  I checked again on the wiring schematic and also did not find anything that would indicate any kind of plugs under the dash.  Again I am stumped why the front lights work great and the back are screwed up. 

[VileZambonie]

Did you check it with a test light?

[bake74]

     I checked everything with my fluke multi meter.  Will a test light show something that the meter won't ?

[1979C20]

It was the plug the runs along the collumn for the blinker switch.

[bake74]

     Well I figured it out.  Just like I always say, check the easiest things first.  My rear metal ground strap that I had taken off and cleaned the ends and where it hooked up too, was intermediately making a connection and then loosing it.  Weirdest thing I've seen, but I did get it fixed and was able to pick up the christmas tree last night.