First thing to spring to mind would be silicon sealant
run a bead round the rim and push together, as for water proof wiring connectorshttp http//www.vehicleproducts.co.uk do a good selection, but what could be better is to run a continous wires from the inside of the lamp high into the vehicle before having any connectors
Heat shrink is a good method of sealing the actual connectors when made up, put some silicon in the sleaving before shrinking it would do the job