It does it constantly. I noticed it while I had the drivers door open working on the "guts".
The key in or out of the ignition makes no difference, I can't tell you for sure right noe if it does it with the key on, engine not running (acces. position).
I pushed the door jam dome light switch in and it still does it.

I wasn't ditting in the seat, do these trucks have the seat belt warnig? I know the SuperDutys do. You might be on to something here!

It definitely has a MUCH slower rythm than any indicator I've ever seen/heard, at least a 5 sec. interval.

I'll have to shut the door with the window down and see if it eventually stops.
I've been too busy at work (I'm just getting home from work now) and fixing the other little problem that I ALREADY KNOW how to fix that I have spent almost no time on this problem so far. It doesn't seem to drain the battery at all, but this truck starts so easy it's hard to say for sure.

Thanks for thinking of my LITTLE problem! And it's not even an IDI!
I'll bet if it was a Dodge though you would pull my post! LOL