Every valve I've ever seen is somewhere between the back of the livewells and the transom (and usually in the center of the boat since the livewells are). That narrows down the "where" quite a bit. Next, look at where your livewell thru-hull fittings are - that's probably bottom edge of the transom, further endorsing where to find the valves on the other side of the transom. Lastly, start removing things to get to the "suspect area".
I would suggest not pulling out the piece of cable at the console until you check whether you can see/reach it from the valve. It may be a good idea to try to tape a string to it. Feeding cables through some hulls can be problematic without a pull-string.