The convertible is still the car, not a convertible kilt(?!), GP is saying with Scottish as opposed to Californian weather there would be less changing. Say, a single open-top-weather period (two changes) per year. Or none.

(I imagine the Californian poster is changing twice per year too, just using 5-6 ties. With that reading the joke is Hey in Scotland you just keep it closed, never need any more ties.)