Mac,
Your post indicates your truck probably has a key start starter. Battery power is supplied to the starter motor by internal switches that are "made" when the solenoid is pulled in.
The wiring from BATT to starter would affect the ability of the starter to turn, while a problem with the smaller, interior wiring could make it difficult for the solenoid to pull in and spin the starter.
I'm not clear which of these problems you have...with the fan motor on, the starter pulls in but won't turn, or, the key won't even pull the solenoid in.
As I mentioned earlier, the little blower fan would have to pull a horrendous amount of current to draw the battery down to where it wouldn't start the rig...fuses would blow and wires would melt! This is a very unlikely scenario. Good luck, and don't you just love those in-laws.
Stuart
