That's a PWM (pulse width modulation) dimmer.....a bit more modern (and efficient) than a simple pot. I would still confirm it does what you want before cutting/drilling any metal, but that should work fine.
You need PWM style dimmers for LEDs or they become very touchy. A little change in voltage goes a long way with LED brightness and diodes are not always consistent so if you have multiple lights on the same circuit they will be at different brightness.
You could "fish" one of those small pots up into the pillar through a small hole. I've installed trailer hitches in two cars that came with spring steel wires for pulling bolts into frame members. Leave long leads on the dimmer so you can do the connections down low.
The dimmers in the link I offered are wireless/battery powered. You can put the controller anywhere and then the dimmer anywhere, could likely just drill a hole in the plastic trim and tuck it in an existing body cavity. I also wouldnt feel too bad about taking a step drill to the B pillar and drilling a 1" hole. Thats a lot less intrusive than cutting a big square for a junction box with sharp corners.
I use this one with my LED strips: https://www.superlightingled.com/dm1...er-p-3136.html
It's surface mount so might not work if you're going for a super clean look, but it's PWM and runs quite cool, and you don't have to cut any holes. It doesn't technically have an "off" switch but at the fully dimmed position the LEDs are no longer lit and the power drain is low enough that it hasn't been a problem for me. If you routinely park the van for months at a time you might want a separate off switch (although in that case you're probably going to be using a battery disconnect anyway.)
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N8SRE
1990 E-250 Sportsmobile w/ penthouse top, converted when new by SMB Texas.
Vanna
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CSC TT250 + homebuilt trailer
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