[dmjlambert]

I think that sounds like failure of the solenoid to engage. It can be caused by a number of things, I listed some in my earlier post. The easiest thing to diagnose would be weak battery if you have another vehicle's battery to swap with.

You didn't mention the condition or type of your battery terminals, those are suspect. Man, I sure hate clamp-on emergency repair terminals, but they are common. You did mention electrical problem of unknown battery drain, I don't know if that is directly related to the starter problem, but it could be. That really needs to be figured out.

You can also attach using an alligator clip test wire on the purple solenoid wire where it attaches to the solenoid, and measure the voltage with a multimeter during your attempted starts. Be careful with the placement of the test clip, because it would be easy to short across the other terminals and cause the starter to turn. So, disconnect the negative battery cable at the battery while you are placing the test clip. While you're there, make sure the connections are clean and snug. If you have a large voltage drop when you are attempting to start, that could indicate a problem with the connections and wiring from the battery and into the cab and through your ignition switch and back out of the cab to the starter.

Move the multimeter to measure the voltage at the battery, and see how much the battery's voltage drops during an attempted start. If the battery voltage drops just a little during an attempted start, but the voltage drop on the purple wire where attached to the starter is big, the problem is with connectors or wiring. A common culprit on these trucks is the connector on the back of the ignition switch being over heated and melted. So, look up under the dash and inspect carefully.

There are several connections to check, examine all connections from the battery, through the connection block on the passenger fender next to the battery, pull on the fusible link wire to see if it's fried inside the insulation, check the wiring on both sides of the firewall connector and inside the connector, the fuse panel only attaches with 2 screws so it is easy to move out of the way to check the wiring underneath, check the connections at the neutral safety switch. I'm talking about 12 gauge red wires going from the battery all the way to the ignition switch, and 12 gauge purple wire coming out of ignition switch and through the various connectors leading all the way to the starter.

While you have the firewall plug disconnected, pull the wiring and connection block out of the driver side fender and have a look at it. That is a great place for rats to have had a snack of your wires at some time in the past, I have knowledge of that.

If you're not finding big voltage drop anywhere it could be your solenoid needs replaced. Others on the forum may have more advice, and possibly better. Patience and checking stuff...

Sorry about the long winded post. Lots to say I guess.