I was just looking at your location thinking you're around the corner from me, but you're not... I almost lived in an apartment in Media about a year ago... Now I'm up on the border of Chester and Berks.
Since I seem to have started selling/giving/trading these trailers a lot (I will stop when I find 1 that's perfect for me...or 2), I've gotten changing to 12v down to a 15-20 minute science. 10-minutes on-site if I pre-make the trailer harness. Running new lights and wire is the
hard way IMO...
You pop the lens off the military light with a drill and flat head bit, swap the bulbs in, then crimp on connectors to a flat 4-wire civvy harness, and you're done. Spend an hour if you want to make it really nice...
The first one I did was pretty sloppy, I sold that trailer, but wish I could go back and re-do it. Since then, I've started making the harness ahead of time, which takes about 15-20 minutes to do it nice with wire loom and sealant tape. I have 2 or 3 on my workbench right now.
One small way to save $3 and 15 seconds... The top bulb in the military can is the running light, and the one below it is the brake light (bottom two are completely unused in civilian applications, consider them spare running light bulbs). If you leave the top bulb military/24v and just swap out the one below it (or move the one below it into the top position), the lower civilian voltage will light the mil bulb at a running light brightness, about the same as a smaller 12v light. I may catch flak for that suggestion, but I've had it next to my civvy pickup and commuter car running lights, and it's the same brightness. Changing that bulb to a 12v bulb is not necessary. Plus, a 24v filament will last longer when run at 12v. I do the same thing at work with expensive fixtures, buy the 240v bulbs and run them at 110v (not good for CFL's, btw).
I believe M101's are under the required size for marker lights, but I'm going to start adding them to trailers I convert. I always thought they help the driver at night, and my brother in law cemented that idea to me when he said the same thing as we hooked up his M101 for a late night tow.
Hard to tel in the pic, but that's wire-loom around the 4-way wire, not a cut up mil harness. Although I have so many mil harness's, I did start cutting them up and adding 7-way flats to stop using an adapter on my tow vehicle.