Constant 12v power to truck bed

[Bwarne1944]

Hi all,  lets get this out of the way first, I know very little about wiring.

I have a 2010 Chevy Colorado.  I have a topper and would like to hook up LED light strips inside that can be turned on and off from a switch.  Ive purchased the light strips and they come with wiring and switch.  My truck bed has wiring already that is hooked up to the topper brake light and interior light (though I don’t think its worked for a long time). It seems like there is only power when ignition on.  My question is:

 

How or can I get constant 12v power through that existing wiring that can be controlled by the LED light switch?

 

Thanks in advance

Brad