Hey jdmckay...I'm no expert on such issues, but might be entertaining until someone shows up to give you good answers.

Electrical problems are NOT my forte, which is why I try to wire things like pre-amps and auxilary circuits on their own, new circuit line, rather than tap into "mystery poles". As your truck is some 27 years old, an ignition switch might be suspect.

Woops, posting this and I see Geezer showed up. Thank gawd!~