Fabia 2016.

Havent seen this before , it's around the back door.

No warranty efter, what to do? 

 

Can't quite orientate myself just where this is, but if there's no warranty I'd be inclined to clean it thoroughly with Methylated spirit or some such and try a CA gel (superglue) being careful not to glue your fingers to it!  They do various viscosities and a gel being thicker might be best.  Should that not work I don't think you will have made the problem any worse.  Good luck with it.

Or could try something like: https://www.screwfix.com/p/geocel-the-works-sealant-adhesive-black-290ml/66022 ? Flexible, waterproof, black. The hinge on my my fiesta's boot lid started leaking, slapped some of that stuff around the hinge and was dry for 3-4 years after. Not saying this is the right way of sorting it but hey, I don't see why it wouldn't work and stop it from splitting further 🤷‍♂️

It looks like it needs bridging in the gap interior, to stop the split spreading, then gluing and filling.

You will have to do some research, try contacting Loctite products they used to do a multi plastic glue. 

or....fit a new seal.

My Skoda-dealer pays for it, for free👍

The design on the door seals is a bit odd the the fabia, first car I've had where some of them are visible/sticking out on the outside of the doors. Seems destined to get knocked and banged about, baked in the sun etc.

 

Looks like there are a few scrapes on yours which may of been how It started I guess.