Like many here, I have been less than pleased with the view offered by the stock mirrors. I picked up some cheap bar end mirrors off of Amazon and installed 'em. The results were... well, good & bad. I hope this helps the community. This is for a 2016 S.
I bought these:
Astra Depot 7/8" Universal Oval Handlebar End Side Mirror Rearview Light Brown Anti Glare. I picked these because the price was right and they looked angular enough that I thought they would go with the aesthetic of an electric bike.
First step: pull out the plugs on the end of the handlebars.
Next, select the proper expansion sleeve. In this photo, you can see both sleeves. Ignore the larger one, you want to use the smaller.
The trick is to tighten the nut to the point where it is held tightly against the expansion sleeve and starts to expand. But not so much that it expands more than the interior of the handlebar. Stick the bar mirror end, expansion sleeve first, into the bar end and then tighten with a 5mm allen wrench. A wrench is provided for you, but it's small and sucks so I used the one out of my toolset. I also had to hold the black portion of the end cap still in order for it to expand properly.
At some point, the allen wrench will stop turning and you have a black ring sticking out of the end of your handlebars. Tighten it up a little more. Quite naturally, the mirrors will attach onto these rings. Use a 3mm allen wrench to mount the rings, position the mirrors and tighten everything up. Everything on the mirror assembly itself takes a 3mm allen wrench.
When you're done, you can take the old mirrors off to get rid of that bug antenna look:
So... neat and all, but the mirrors are tiny. Here are the new mirrors up next to the old mirror.