I had one of those heaters that plug into the lighter socket hooked up, when it suddenly stopped working. Even the regular cigarette lighter won't work. So I figured I'd just check the fuse, but the fuse is good. Is there a second fuse somewhere that I am missing?
There's a safety thermal protector built into the back of the lighter socket itself, put there to keep people from jamming the lighter element in (past when it's gets red hot and should pop out automatically) and setting the car on fire. You have to replace the entire socket "cup" assembly, the thermal fuse is not available as a repair part.
Of course, it'll also pop if you draw too much current from the lighter socket for too long. Running a 12V electric heater that's drawing 20 to 40 amps for a long period certainly qualifies.
Get your regular heater fixed, and/or put in a separate accessory 'lighter' socket wired straight to the battery with heavy wire that can handle the current - the factory lighter socket can not. Or get your factory lighter socket replaced and throw out that 12-volt heater.
The small wire to the factory lighter socket is only good for about
10 amps in short bursts, much more and you risk burning up the whole wiring harness - do you want to spend $1,000 to $2,500 fixing the truck, or scrap the truck as uneconomical to repair after the electrical fire under the dashboard?-->--
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