i checked mine today and the reverse bulb has a 501 fitting but the bulb is approx 30mm in length,maybe if you use a canbus led as they are longer and they also have the leds positioned on the end
to throw light outwards maybe something like these
http://www.ebay.co.uk/itm/10X-T10-8-SMD-501-W5W-ERROR-FREE-XENON-WHITE-CANBUS-LED-SIDE-LIGHT-BULBS-/190929841243?hash=item2c744e2c5b:g:7M8AAOSwEK9T30oR
http://www.ebay.co.uk/itm/Compact-Star-XENON-LED-Parking-Side-Light-Bulb-12v-501-W5W-T10-wedge-canbus-/290731801199?hash=item43b0f73a6f:m:mNXdXgMicyB10H0mBLUGkWw
I used one of these. The bulb is a W16W. I ordered both but one of them was a little to long but I cannot remember which, sorry not very helpful. The COB led on the end is very bright.
http://www.ebay.co.uk/itm/171944953381?_trksid=p2057872.m2749.l2649&ssPageName=STRK:MEBIDX:IT
http://www.ebay.co.uk/itm/291324868533?_trksid=p2057872.m2749.l2649&ssPageName=STRK:MEBIDX:IT
Ed.
Having looked at your links, the first one, the LED appears to be 45.5mm long and the second one is 48mm long. Am I correct in thinking that the longer one doesn't fit anyway?
Thanks for the links. Are you still happy with the end result?
The Led bulb is much brighter than the filament bulb it replaced. It almost makes up for there only being one lamp fitted. I would post a picture but on the train at present on my way home.
One of the bulbs in the links fitted although I think it was the girth rather than the length but to be honest cannot remember now. I would have a look but it involves removing the boot lid liner trim which is a pain.
Ed.