E30 Zone

Where's the best place to fit waxoyl? If you strip out the interior carpet can you get access to the sills without drilling them?

Easily.

Assuming the convertible is basically similar to the 2DR, you shouldn't need to strip out the carpet top access the sills, just remove the plastic sill plates on each door and pull back the carpet at the side. If there is a self adhesive strip over the inside edge of the sill, pull this off carefully as it is easily broken. To get access at the back, remove the inside rear quarter panels(rear seat cushion needs to be taken out first). Get a long flexible lance to endure to can reach along the inside of the whole length. Put some paper on the floor under the sill as the wet wax will run out the drain holes making a bit of a mess.
You could try spraying up through the drain holes but it's very difficult to know what area you have covered.

Removing the carpet gives you the ability to remove the bungs for the chassis rails and treat in there too.
Well worth doing, although the paraffin smell lingers for weeks afterwards. Don't put too much in the cills or doors as you may block the drain holes unwittingly.