It's Hot out here

Now comes the fun part. Clean the vinyl trim and lay it in the sun (get it HOT). This is Vegas and 105 is normal. Back the car into the afternoon sun. When the painter's tape starts falling off, you are just about warm enough.

Angle cut one end for the miter and begin easing the molding on to the base. I don't think this would work cold.

Ease one side of the molding on the base and push over to snap in place. I recommend pushing back towards the starting point as you go in case some shrinkage occurs in cold weather.

Cut your miters carefully 1/8-1/4" long. Push back to fit and it will expand hard into the 90 joint.