I found a thread by Black Lab on installing a 12V outlet,
which in retrospect seems far easier and more clever than this. But this is what I did. Maybe it will help someone get over the fear of cutting into their dashboard and "just do it."
The usual suspect: a marine 12V "cigarette lighter" power socket:
Note I've soldered wires on the connections and shrink-tubed them BEFORE starting the job. Soldered because I don't EVER want to monkey around under here replacing a popped-off spade connector; done BEFORE the job because I don't EVER want to monkey around under here soldering. Note also the tabs every 45deg on the locknut.
The candidate area covered in masking tape: this will allow marking, re-marking, and cutting without damaging the surrounding plastic. To the right you can see a handy metal hoop into which you could likely just shove the socket and be done a-la Black Lab.
The masking tape allows for banal marking methods. Make sure you have clearance BEHIND the panel to accomodate the penetration of the installed socket and it's wiring.
Note since the nut threads on the socket, the inside of it is a good approximation of the socket's size.
The plastic is soft, and about 3/32" thick (guessing). There are MANY ways to cut a hole in it. A utility knife + patience is a viable option. I think trying to buy the RIGHT sized drill bit here is a mistake/ overkill. Fit is really critical, drill walk would be catastrophic, and it's just a 3/32" thickness not wholy unlike the consistency of hard cheese.
I drilled a bunch of small holes for some reason.
I thought the slug would pop right out, but I needed to connect some of the dots with a razor blade.
This is the part where I actually paid attention to my work, so I didn't document it well. Note the file in the center right side of the pic. Happily it is rounded on one side and worked a treat to smooth out the edges and sneak up on my marked line. Checking fit often, I stopped when I could JUST wiggle the socket in.
Again, soft plastic - many ways to skin this cat. Sandpaper wrapped around a dowel - or even a marker - would work well, too. Don't over-think this...
My carefull fitting resulted in having to actually "thread" the socket in. I think the socket plastic is a little harder than the dash plastic and it tapped the hole (I had to turn it all the way down and it bottomed tightly).
Thread the nut on the wire, and the wire guides it right to the back of the socket. I don't think you need to be much more than hand-tight on the plastic nut, but the plastic tabs enable a little you to shove it around with a screwdriver - meaning you only need access to a few degrees' worth of nut if you have girl hands. You can get this on in a VERY tight space if you're patient.... and have girl hands.
I fed the wires under the tank along the left side frame, using zip ties as it seemed prudent. There's plenty of room and options for routing.
The money shot... taken maybe 15 minutes after starting the job - altho it was wired previously per a post I saw somewhere here, which brings me to my last potential, "hey, why didn't I think of that?" moment:
The battery end.
I had previously wired the other end of the connector to a spare battery tender connector (so I could use it in my tankbag until I had the time to do this) and just couldn't think of a compelling reason to cut off the connector and install lugs.
Ok, hopefully that provides a few idea and some motivation. Just do it. Unless you go at it with power tools, insanely large cutting tools, or personal clumsiness amounting to slapstick, you'll be fine.