See attached pics... I've got a little leak in my roof that I think is coming from the passenger side front corner. I can see some deteriorated rubber seals behind a little piece of trim. I think if I get some silicone sealant in there, it should stop the leak. To do a cleaner job, I'd like to remove a little piece of trim, then seal and replace the trim. I wondered if anyone knows if the little trim piece just pops out, or if there's some other trick to get it off without breaking it... I'm guessing it's probably designed so you have to drop the front bumper and/or subframe, right?

Here's the trim piece I want to remove/replace... do I have to drop the front bumper or anything first?

I think this is where it's leaking

 

If I recall correctly and If I'm not mistaken; the vertical black shiny pillar/column trim along the windshield (below the the piece you want to remove) needs to be removed first.
You will need to pull back the upper door seal to access the screws. Be careful though as there is one screw that is kind of hidden at the bottom that (I missed) can be easily missed. Be sure to remove it or you will crack this pillar.
The pillar is then held in by plastic "push" clips along its vertical center. Just give it an even gentle pull and it will pop right off.

EDIT: Questioning my own memory - I found this video: ...you can skip to 4:01 for the quick

 

Be careful. That corner bit is not available on its own. It comes as part of the whole rail trim that goes back to the rear. It has a clip, and mounts into a hole. We had a leak there. Black 3M trim
adhesive will work. Just be careful pulling it apart.

 

I wish I had been smart and asked about this like Brownshoe did.

After replacing my A-pillar trim after the windshield installer finished it off (it was cracked before he installed the windshield), that corner piece wasn't sitting just right and in my attempt to remove and reset it, I broke that tab the clip slides into...and cracked my new A-pillar again.

When I priced it at the dealer, that's when I learned it's only available as the entire side section. For about $125.

 

If your MINI has a sunroof the leak might be from there. The hoses on the drain tubes often fall off and leak onto the fuse box or BCM which can lead to electrical problems. The hoses should be reconnected to the sunroof tubes with small hose clamps. You should be able to access the tubes without dropping the entire headliner. There are also drain tubes/hoses in the back which should be secured.