Became interested in comparing the relative merits of the location of side-mounted turn signals/blinkers/repeaters. This was prompted by the new Buick Regal and TourX wagon (Opel Insignia?), which have these lights mounted on the fender quarter panel.

Typically these are little LEDs or bulbs at the outside edge of the wing mirrors. Classic BMWs and Saabs had them behind the front wheel on the front quarter panel. Older Volvos in the US market can be retrofitted similarly. Personally, I prefer it mounted on the sheet metal. When I see one of those 'classic' repeaters, it stands out without ambiguity. It's just more visible to my eyes.

But on many (most?) cars, side-turn signals built into the mirrors are less visible when viewed from a rear angle. Sometimes, the mirror-mounted blinker seems to float in empty space disparately from the car on which it is mounted. And that makes me lose my focus on traffic. "What's that yellow light over there in the distance?" The mirror-mounted ones seem less functional and more like a decoration.

The quarter-panel/fender mounted one is unmistakably anchored to its vehicle, and to my eyes has clearer, uncluttered 180-degree coverage. I gather it also has the advantage of being cheaper to maintain, making for simpler, less expensive mirror housings.