Dig out the Fuse Diagram leaflet. Often its placed in with the spare wheel kit.
If your cigar lighter stays on when start stop cuts in then go for a cigar lighter/power socket fuse.
Note on mine I have front/rear cigar lighters ( despite having no smoker packs, and a boot power socket. The rear and boot react differently to the front as to when power stays on e.g. the back 2 only get power when the engine is on! So you have to check.
If they are 15A just downgrade the first fuse to say 10A, and add a 3A for your cams.
Thats what the main dealer recommended. ( note it then does not matter which way the pigtail goes as it will never pull more than 13A ... so you are safe.
On my S Class Coupe I went for a fuse in the Cab ... as the route from the engine bay looked nightmarish. No room to fiddle around a 5.5L V8. Be lucky to stick a knitting needle between component, and their are “intake” fat tubes/turbos?? ( not a mechanic) that REALY heat up when the engine gets to just normal warm! So if you do go in there check first when she is hot.
It wasnt too hard getting into the boot by removing the rear boot vertical wall. But on an S Class the panels are REALLY tight so I went for the “route of least disturbance”.