I know a bunch of switches for GMT800s (well, at least a bunch of them as described in my '04 FSM) go to ground when the switch is closed. The switch for the cargo light isn't directly part of the cargo light circuit, the switch just grounds a pin on the bcm, and then the bcm powers (or grounds) the circuit to the cargo light, resulting in it turning on.

Actually, I'll dial it back to "it probably goes through the bcm", as I haven't gone through the wiring diagrams for the cargo light, so it might be an odd-ball direct-wired setup, but most lights go through the bcm.

 

Thanks davester, makes sense. Hopefully I can find a wire that powers the cargo light off the bcm, otherwise I'll have to drop the headliner down a touch to tap into the cargo light circuit at the back of the cab itself.

 

No, no, no..If you want the cargo output, connect to the load side. Iirc, it'll be in the mbec. Don't have schematic right now, but, will later.

 

I can not for the life of me find the load side wire in the cab. Pretty sure I light tested every wire on driver side and pass side.

 

You couldn't find it because the feed to the cargo lamp is always hot (technically 10 minutes hot, after key off, called INADVERTENT POWER). The cargo is ground controlled (the ground isn't timered). It is in the MBEC (junction panel adjacent to park brake pedal).
It should be in either the 6 or 12 way HDLNR connector.

Technically, you could tap the blue/ white at the cargo switch, instead of in the MBEC. Since the ground is controlled, your coil side of the relay that you'll need should be on the inadvertent power.

 

ok thanks