[ray_mcavoy]

Was the 6V measurement on the purple wire taken at the end down where it attaches to the starter solenoid?

If so, try measuring it again on the other end of the purple wire right off the back of the ignition switch. If you have full battery voltage (or close to it) there at the switch, but it drops down to around 6V at the other end of the purple wire, there are a couple places to look for a poor connection that's causing the voltage drop:

(1) If your truck has an automatic transmission, check the neutral safety switch. Try shifting to neutral (instead of park) and see if that makes any difference. Also try moving the shift lever a little while attempting to crank the engine. And finally, for testing purposes, you can temporarily bypass the switch by unplugging the 2 purple wires and connecting a jumper between them. Don't leave it this way because it will allow the truck to start in gear.

(2) The purple wire passes through the bulkhead connector on the firewall (located behind the fuse panel) on it's way from the ignition switch to the starter solenoid. A poor connection there could be causing the voltage drop.

However, if the voltage is also measuring low on the purple wire right off the back of the ignition switch, you could be dealing with an ignition switch that's getting bad and not always making good contact. But one last thing to check before replacing the switch is to measure the voltage on the red feed wire to the switch while attempting to crank the engine. If the voltage on the red feed wire is low, there is likely a bad connection somewhere in that circuit between the ign sw and battery.