Wiring Diagram for Cargo light switch

I think my 1999 is a cross over. It is an OBS and has internal parts stamped 1998 etc. So it is a bit odd.

I plan on using my cargo light switch as just a switch to turn on the cargo lights and bed/rear lights.
Right now there are four (4) wires from the switch and the cargo lights only come on when the dome lights are on. All I want to do is use it as a switch to turn on the cargo lights, and send a signal to the relay that controls the cargo bed/rear lights. This should be independent of the dome light being on.

Do you have a easier option that to remove the third brake light and do a circuit test to figure out which wire goes to the cargo lights? Plus will I run into problem with the BCM if I alter this switch?

Many thanks!