Roof vents

Hi
The best way is probably to use NON setting sealer rather than silicone sealer which can get hard and leak or Sikaflex which may be difficult to remove , if you needed to change it in the future.
The vents that I have used previously are fitted by bedding the top half of the vent (i.e. from the outside) into the cut-out.
Then from the inside, the bottom half of the vent which contains the flyscreen etc., is pushed upwards and matches the top half which is now sitting in the hole. The two parts pull together as the screws are tightened gradually, all round. ?This pulls the top half evenly into the bed of "mastic" ensuring a watertight seal. There are several non setting mastics available
I have used CARFAX CARSEAL IL 99 which is easy to get and use and can be removed with white spirit
Charles Camping stock it and I have seen it in one or two other places, but unfortunately I can't remember where. It may possibly have been in the marine supply shops but not sure.
I'm sure Cosy Classics in Rathcoole also do this or a similar product.

Its a simple, half hour job, the most difficult part will be cleaning the roof of the old sealer.
Depending on how much sealer is present, you might use a paint scraper, then a Stanley knife blade (removed from the handle to allow it to go evenly onto the roof) and then a solvent to remove the last bit. I'd try white spirit or turpentine or perhaps petrol, or there may be a special solvent. How well this dissolves the sealer will depend on the composition of the sealer, but it shouldn't be a major difficulty to clean it without any special solvents.

Sorry I can't comment on the merits of Heki vs. Thule

PS challenger, I wouldn't trust the mastic strip due to the fact of the extra risk of water ingress during driving. The reason I went for sikaflex is that Ive used it on boats for years and its never leaks if used on a clean surface. Also no whistling or drafts to report so far!