if the rubbery foam looks like its tired or deteriorated badly and its replaceable its prob worth finding some good cond 2nd hand or new ones
if the rubber can be taken off the light I would as stated above remove the rubber and give it a liberal spray on the fixing side before refitting of this
http://www.ebay.co.uk/itm/AUTOGLYM-S...item1e701537e3
or give it a smear of grease along inner surface channel - can use grease on outer edge as well but silicone is a bit easier and not so messy - silicone usually works for minor leaks or grease if its more major as its that bit thicker
quite often the rubbers just get old, dry out get hairline cracks and get a bit stiff and brittle - a good wash with hot water and fairy liquid followed by a spray with silicone or wipe with grease will do the trick and its cheap and easy to try