underbody coating

Land Cruisers are very good at rusting, especially if you drive on salted roads. The best rustproofing there is is rubberised undercoating, but that can only be applied on brand new vehicles, as it doesn't stop rust, it just protects against salt and water. I decided to try some options for used...

Just had the underbody coated in Waxoyl, and asked dealer to also apply the 120-4 Cavity Protection but they refused, stating they did not want to damage the door electronics and get product into cabin. I am thinking of applying this myself into the doors, hatch and hood. Does anyone have any...