Answers to your questions:
1. Is your turn signal a clamp-on aftermarket switch (part no.? and brand?) or a GM accessory switch which is part of the column "bell"?
GM accessory switch part of steering column.
2. Does the turn signal switch have 6 wires going into it?
Yes
3. Describe what happens when switch lever is push down for left turn. Describe what happens when left lifted up for right turn. What do front and rear light do or not do?
With all wires connected, when switch lever is pushed down for left turn the left front turn signal flashes normally. Left rear turn signal barely flashes and uses filament in bulb that tail light would use. Bulb is 1157. Verified that wiring to light is the same as right side that works OK.
When lever is lifted up for right turn, all lights work normally. Right rear light uses filament to indicate turn signal, tail light uses other filament. Right front directional works OK. No change if headlights/park lights are turned on.
4. Do brake lights work properly? Do tail lights come on with headlights?
Brake lights work. Tail lights come on with headlights and work OK until left turn signal is used (as explained above).
5. Were all lights and turn signals and brake lights working and this just happened? If not, explain history of this issue.
Truck has been in storage for several years. I had previously driven it but can't remember any issues with the lights. However, it wasn't a daily driver and used primarily during daylight hours at the time so might not have noticed any issues. As I said, can't recall.
Please don't guess on any of this or use previous observations, please test again. Describe carefully.
