Cracks in plastic window frames

We have a 2005 Symbol which we've owned for 12 years and had great service from, and fun with.  We're pretty careful about keeping her in tip top condition. Over the last year or so we've noticed small cracks appearing in the plastic window frame moulding in the shower room.  I've covered them with insulating tape as a temporary measure to prevent water from the shower getting behind the panel, but would like to make a rather more permanent repair.  Does anyone know of someone who might be able to replicate the moulding, or have any other ideas on how to repair this? I've been in touch with the spares department at Willersley, but as expected, these are long since out of stock. I suspect the cause is simply ageing of the plastic exacerbated by heating and cooling from the sun. Since we intend to keep the van for at least another 12 years or more, we're prepared to make the investment in a permanent and good quality repair.

At least as a temporary measure if they're small cracks, see my response to a similar question last week:

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By the way, I love your Hardy-inspired handle! 

I'm absolutely certain they are 'stress-cracks' usually in the corners, if you were to hold the frame in both hands in front of you & push the right hand up & left hand down, the corners of the plastic molding with fail eventually by cracking, this is exactly whats happening in our vans, mine included.
Our bathrooms are behind the rear axle, & as such, it can 'bend' the whole rear-end slightly whilst travelling,  even though there are extensions/outriggers, a fraction of a mm over the years on our billiard-table like roads will cause a failure. I bet if you look around the washroom you will see witnesses of the wall covering 'creasing' in places also, its all down to very slight movement.
Captain trollies fix is far too thin & simply runs away on most cracks, as mentioned earlier, the best way is to remove the frame, clean & abraid the corners etc & re-inforce with a little grp patch.
I suppose I SHOULD have a go at mine as well, but can't be bothered at the moment, I will one day, honest.