hi al!
i ran into the same issue a couple months ago. the mirror, though awesome, will require modifications to work on the radrunner.
note: be very careful not to lose any parts inside handlebars!
(1) grind down both ends of the "barrel nut" equally until the nut will slide into narrowed section inside handlebars (this will take quite a bit of grinding to achieve the correct length). you'll need to remove a large amount of material from both ends - check often during grinding process as to not remove too much material.
(2) chuck the "small wedge" in a drill (this will take a little ingenuity - i used a piece of bamboo skewer wrapped in electrical tape). use sandpaper (or similar) to reduce diameter of the wedge while spinning with drill - test fit in handlebar frequently during this process being careful not to remove too much material.
(3) loosely assemble mirror components. align wedge pieces and apply small piece of cellophane tape to where wedge pieces meet to keep parts aligned. slide into handlebar and tighten per instructions.
good luck!
- will