Courtesy Rock Lights

Hello,
I have been looking at installing rock lights and found DSIM’s plug and play kit, which allowed the rock lights to function as an extension of the third brake light “cab light” switch. This allowed the rock lights to be manually turned on with the on position, or used as a courtesy/door light with the middle position. From what I gather it seems DSIM no longer is producing these parts and they are out of stock. Does anyone have any experience
recreating this set up, or would anyone be able to point me in the right direction if this has already been addressed? This is my first lighting project and I have no experience with vehicle electrical systems. TYIA

DSIM product for reference: https://dsimsolutions.com/pages/rock-light-harness-bundles

 

The DSIM gadget isn't necessary for that.

 

Right, just looking for a point in the right direction. Dont have experience making my own wiring harnesses or connections. Have only used plug and play