What's the best way to install taillight wiring from a rear-wheel bottle dyno to an LED taillight mounted on the rear fender? Glue, tape, use the fender internal metal as power or return leads ... ?

I have Zefal plastic/metal sandwich fenders.

I would find some heat-shrink tubing that matches the color of your fenders, run both wires through it, shrink it, and attach with small blobs of silicone. By using a hot lead AND a ground wire, you will pretty much eliminate the problem of flakey grounds causing problems with operation.

Silicone, you mean RTV? I wouldn't think that's strong enough for a pretty permanent installation.

On the taillight, I'm planning to run the return current throught fender stay, which is bolted to the dropout and seatstay, where the bottle dyno is mounted. I'm not real worried about the tail ground, though the return wire is the most reliable approach. But on the headlight, the ground runs through tthe headset, and I think that's perhaps a little less reliable than the chainstay, even though it works now.

A 'dollop' of silicone when dry will be pretty permanent. As far as 'frame ground' vs ground wire, you will only find out how good your frame ground works when you are miles from home, and the lights start to mysteriously flicker. As a new install, I would run the 'extra' wire for longevity's sake. And peace of mind, of course.

What about attaching the wire on the tire side of the fender rather than the outer side of the fender?

tire side is traditional, usually there is a lip that will allow extra gluing surface. I would also run two wires, I really don't think you'll be happy long with frame return. The zefals aren't made to carry current, are they? Some of the old Esge fenders had conductors molded in.

There are three metal strips embedded in the plastic, but no provision is made for termination to the light or to external leads that would jump to the gen. I'd have to carve off a lot of plastic, with the risk of finding metal that is hard to solder or just plain fragile.

I just took off the fender. I'm gonna give the inside a good wash, drill access holes, mark the gluing area, and prepare to install a pair of wires using RTV.

Wife's Breezer seems to have turned-in lips which can be used as conduits for taillight wires. Not so the Zefals!