I am looking for a switch, where I could use a brake-tail light as a low/high intensity lamp for inside my cargo trailer (I need red).

Are they two separate circuits, or is there a double pole switch that will energize ether tail (low), or brake/tail (high)?

 

So you want to reach into your Jeep, flip a switch, and have your brake and tail lights come on? And you want to choose either one, or both?

That's possible, but you'll need more than just one simple switch.

You should never just apply power to an existing circuit from a different source, like running 12v to a switch and then splicing the output of that switch into an existing circuit. Bad things can happen. You need to set it up so the original circuit is isolated. (Like I did here with the fan switch.)

For that, the easiest way is going to involve some thinking, and some relays.

If you are comfortable with electricity, you can design a setup to do what you want. If you are not, I suggest you either get help from someone who can, or just find a red light at an RV or surplus place or something and wire that up separately from your brake/tail lights.

http://www.the12volt.com/relays/relaydiagram25.html