After helping a friend through this experience with his 05, I think I'd go with a simpler [cheaper] solution. If the secondary solenoid fails in the closed position, its probably because it welded itself closed. If the solenoid feed wire [not the ignition wire] is the target, think I'd go with a "strategically" placed set of wire nippers or a properly sized butt connector. "KISS" lives at my house......
PS--I do like the idea of switching the feed source to a properly fused, chassis switched circuit--wonder why its wasnt OEM?... and the back-up beeper is pretty cool too....
2003 40' MDTS
Garden Ridge, Texas