With only one spot available, I have an idea to shoot your way. (though it might not be worth the effort for just one switch)
Centralized Accessory Override Signal and Control
With vehicle electrical systems getting more crazy and temperamental to modification, I suggest building anything else, parallel to the existing system, only touching it in robust spots, and not drawing power from the vehicles wiring. Tap into the cigarette outlet (or better yet, go to M7 in the fuse box) send that to the switch, along with 12v dedicated, the signal from the switch goes to a main relay (fused directly fro the battery) that powers your extra devices. The switch positions are ON - OFF - ACC (or just ACC - ON), ACC meaning it turns on with your key just like the radio. This is how my far left switch operates, to control the CB and Ham radios.
ACC (M7)------------>switch<----------\/-----------[fuse]-----------Battery +
. . . . . . . . . . . . . . . . . ^ ---------- [relay] ------ Awesome custom electrical system
How to tap into the M7 fuse without using a Fuse-Tap
https://wayalife.com/showthread.php/36813-New-Rear-Lighting-Idea?p=704834&viewfull=1#post704834
