Dennis, I looked up the body wiring diagrams at the WBGO site and they show a 2004 331C (a Minnie 31C) and a 431C (a Minnie Winnie 31C). I saw no 12v wiring shown in the cabover area on their installation dwgs but there was a 110v line feeding a receptacle (on the driver's side I believe.) Even then, it was run in the ceiling and then down to the receptacle on the sidewall so one wouldn't need to worry even on the driver's side when drilling upward. You might want to look those dwgs over. They are at the WBGO Industries site:
then on the left side, click on "Wiring Diagrams-Motorized" to find your rig.
I believe the "end cap" Eagle 5 was referring to is that WBGO (and just about all manufacturers) finally gave up on the huge front window there at the cabover.....like your 31C has ..... too much movement + bird impact, etc, and just prone to leak. The later models just have a window-less cap for the front piece....so to speak. My 31C has side windows, but no front window.
I have to admit I don't know for sure about the screws in that front trim piece, but I had always thought they just went into the 3/4 in side panel. I had to replace the vinyl insert in that trim piece on the driver's side on mine, and I recall those screws (that the vinyl insert piece covered up) and I just really believed that they went into the sidewall panel. I just recall in my mind that the backset on those pre-drilled trim pieces was just about enough to be centered on the sidewall piece; i.e., about 3/8 inch or so back from the edge of the sidewall. I'll look closer the next time I am over at the storage lot........maybe that trim piece is a lot wider than I remember it.
You might give Winnebago Customer Care a call tomorrow at (800) 537-1885 - Toll Free or (641) 585-6939. Ask to speak to a tech ...... they are usually pretty helpful and it's a good service they offer to WBGO owners. Have your rig's WBGO serial number ready when you call (not the VIN) and they will pull up the records on your coach. They have a lot more detailed dwgs and other info at their disposal, I believe, than what is listed on their Winebago Industries site for us for us owners to struggle with.
Anyway, good luck with the project, and safe travels.