This procedure should ONLY be used if you're familiar with a silicone gun and have BLACK or CLEAR silicone to use
1. Lower roof, open boot. Look at the rear of the roof where the drain is, and you'll see a slight intent where the seal touches the Tridion cell. Over time this pushes down further, and the seal becomes less water-tight.
2. There'll be a distinctive line where it touches, this is to be used as the guide. Get your silicone gun out and cut the feeder tube (the nozzle) of the gun to roughly 5-7mm wide hole so there's a wide(ish) line of silicone able to come out.
3. Starting from the end, work your way right across, slowly. I waved mine so it looks like a long ~~~ but a straight, thick line would be fine too.
4. Leave to dry. It's recommended to leave for 24 hours, but in the UK this isn't really an option (especially if you're outside). I left mine for 3 hours and all was well when I pushed it back up - the longer, the better.
Conclusion : It's not necessarily going to stop your car leaking, in fact I know it won't. But it will minimise the amount of water that the seal is having to handle. A small comparison can be emptying a bath vs emptying a shower down the drain. The bath takes longer, as there's more to handle. The seals aren't exactly world class, so minimising ingress is best :-)
Picture and PDF version coming soon.