Chevrolet Repair: Electrical/Ignition 1988 2/4 ton truck, wrie, battery cable

Question
I have a 1988 Chev 3/4 ton with a 5.7 l ( 350 ). It is a farm truck so nothing to exciting. The problem is it will not shut off. I can start it drive normatl, but when I try and turn it off, it stay running. I can remove the key and still stays running. I thought it might be the ignition switch in the steering column, but replaced that and still the same problem. Any ideas? I thought maybe I could put a toggle switch on the power wire to the ignition switch, but I do not know which wrie to coose. Can I find a power line to the distributor that would cut all power? If I leave the truck for any lenght of time with out removing the battery cable the battery goes dead.  

Answer
Hi Dan,
Try disconnecting the alternator, and see if it will shut off with it unhooked. If it does, leave the alternator unhooked for the period of time it takes to kill the battery, and leave the battery connected, and see if it kills the battery.
I think you have an alternator diode problem, which could cause both problems.
Appreciate if you let me know how it turns out.
Van