Changing rear light units...

My RHS teil light/indicator lights have hairline cracks in the lenses and I’ve got some brand-spankers to go in their place. Looking at it, the rear/brake-light shouldn’t be too much trouble, as it’s easily reachable. The indicator unit, being closer to the side of the car is very very hard to reach through the boot. Will I need to remove the boot side-panel to get at it and swap it over? or is it accessible behind the inner arch? What’s the best way to access them? Car’s up on axle-stands at the moment with no wheels on, so is a good time to swap the lights over