I've done this on several bikes with aluminum fenders using Goop adhesive. I've never had a problem, it's a fantastic all-weather, hard yet flexible adhesive. The goop dries to a semi-hard plastic and encases the wire, protecting it from gravel and sharp debris. It's held up in all weather and on all types of road surfaces flinging all sorts of crap underneath the fender.

I first tape the wire in place at many points as I "spot" glue the wire at the points between the tape. If I try to glue it all at once, the wire doesn't want to conform to the curve of the fender and creates big gaps between the wire and the fender. After the glue sets, I remove the tape and then encase the entire length of wire with the glue. It becomes permanent, you'll never have the option of undoing it. The glue is tenacious. See attached pics, taken two and over 1000 miles of dirt/gravel roads later: