Best way to fix this?

If there are cracks, you might not be able to see them. While you have those rubber strips off, it's best to mask off the roof, doors, hatch, etc. and give it several coats with spray sealant.

Then jam that rubber strip back in place. You have to have the end in the right spot for it to snap into place. I like to snap in one end and press maybe a foot or two of the strip into the slot. Then do the other end. Then you can push the rest of it into the slot. That way the strip won't creep.

 

3M Plastic and Emblem Adhesive, 08061, 5 oz Tube

 

Listen to the other MechE, or buy new weatherstripping. It is that simple.

 

You might need new weatherstripping. But I think it likely that the end of your strip is just not quite in the wrong place. It should all snap down. If it's in the right place and won't snap down, then it's shot and needs to be replaced.

Also, I failed to mention that you'll want to let that sealant dry for several hours before putting the weather strip back.

Also #2, when you spray the sealant, go all the way to the tail light.

 

Inspect the end of the strip AND what is in the channel under the end.

Both ends have a different "clip" that is harder to engage.....and disengage.
If the rest of that strip comes loose, then the suggested cement should work but not the entire length of the strip.

I have had those rubber strips come loose on 3 different Prii's.
Twice it was while washing the car. Once in a drive through.

I don't remember EVER seeing a report of that being the source of a trunk leak.

 

It isn't. Because it is intended to be removed to mount a car top carrier.
Except for that intended use, gluing it down should NOT be a problem.
But some spaces need to be left open to drain the channel.

 

Take note, however, that once it comes all the way off.......
It is likely to be loose enough now to come off on it's own.

Were you able to "snap" the ends into the clips ?

 

I did that maybe 2 years back (on advice here). Ours was barely grungy, but worth wiping out with a damp rag, and then I waxed it. These grunge traps...

 

Clip, groove.....whatever.....the ends fit much tighter than the rest of the strip.

On 3 of my 4 C's, once they got to be about 6 years old, the rubber strip shrunk a bit so that it wasn't tight anymore.......
except at the ends. I used emblem glue in a few spots to keep it from flapping in the wind.

A few years ago, replacement strips were available.
Seems like I remember something like $60 each. Probably more now.

 

Found them, two types, depending on with/without sunroof:


SIDE MOULDING. 2010 Toyota Prius Hatchback | Toyota Parts Center Online

Without sunroof, just under $60 USD MSRP apiece.

 

Yeah. Yours are probably fine.